For some time now there have been quite a few testsuite failures on Windows, but that number seems to have increaded recently. Here are the failures from

sh validate --fast

with today's HEAD

SUMMARY for test run started at Mon Apr 30 16:41:18 2018 GMTST
 5:46:04 spent to go through
    6321 total tests, which gave rise to
   18197 test cases, of which
   11980 were skipped

      46 had missing libraries
    5963 expected passes
     171 expected failures

       6 caused framework failures
       1 caused framework warnings
       0 unexpected passes
      31 unexpected failures
       6 unexpected stat failures

Unexpected failures:
   backpack/cabal/bkpcabal07/bkpcabal07.run           bkpcabal07 [bad exit code] (normal)
   backpack/cabal/bkpcabal06/bkpcabal06.run           bkpcabal06 [bad exit code] (normal)
   cabal/ghcpkg05.run                                 ghcpkg05 [bad stderr] (normal)
   ghci/linking/ghcilink001.run                       ghcilink001 [bad exit code] (normal)
   ghci/linking/ghcilink004.run                       ghcilink004 [bad exit code] (normal)
   ghci/linking/dyn/big-obj.run                       big-obj [bad exit code] (normal)
   ghci/scripts/ghci057.run                           ghci057 [bad stdout] (ghci)
   ghci/scripts/T9293.run                             T9293 [bad stdout] (ghci)
   ghci.debugger/scripts/break006.run                 break006 [bad stdout] (ghci)
   ghci.debugger/scripts/hist001.run                  hist001 [bad stdout] (ghci)
   ghci.debugger/scripts/hist002.run                  hist002 [bad stdout] (ghci)
   perf/compiler/T14697.run                           T14697 [[Errno 2] No such file or directory: '/c/Users/simonpj/AppData/Local/Temp/ghctest-4kcp39ag/test   spaces/./perf/compiler/T14697.run/T14697.comp.stats'] (normal)
   plugins/plugins07.run                              plugins07 [bad exit code] (normal)
   plugins/T10420.run                                 T10420 [bad exit code] (normal)
   plugins/T11244.run                                 T11244 [bad stderr] (normal)
   rts/T11788.run                                     T11788 [bad exit code] (normal)
   rts/T14695.run                                     T14695 [bad stderr] (normal)
   rts/T11223/T11223_simple_link.run                  T11223_simple_link [bad exit code] (normal)
   rts/T11223/T11223_simple_link_lib.run              T11223_simple_link_lib [bad exit code] (normal)
   rts/T11223/T11223_simple_duplicate_lib.run         T11223_simple_duplicate_lib [bad exit code] (normal)
   rts/T11223/T11223_simple_unused_duplicate_lib.run  T11223_simple_unused_duplicate_lib [bad exit code] (normal)
   rts/T11223/T11223_link_order_a_b_succeed.run       T11223_link_order_a_b_succeed [bad exit code] (normal)
   rts/T11223/T11223_link_order_b_a_succeed.run       T11223_link_order_b_a_succeed [bad exit code] (normal)
   rts/T11223/T11223_link_order_a_b_2_fail.run        T11223_link_order_a_b_2_fail [bad exit code] (normal)
   rts/T11223/T11223_link_order_b_a_2_succeed.run     T11223_link_order_b_a_2_succeed [bad exit code] (normal)
   rts/T12771/T12771.run                              T12771 [bad exit code] (normal)
   rts/T13082/T13082_good.run                         T13082_good [bad exit code] (normal)
   rts/T14611/T14611.run                              T14611 [bad exit code] (normal)
   /../libraries/base/tests/tempfiles.run             tempfiles [bad stdout] (normal)
   /../libraries/process/tests/T9775/T9775_fail.run   T9775_fail [bad exit code] (normal)
   /../libraries/process/tests/T9775/T9775_good.run   T9775_good [bad exit code] (normal)

Unexpected stat failures:
   perf/compiler/T12234.run             T12234 [stat not good enough] (optasm)
   perf/compiler/T12425.run             T12425 [stat not good enough] (optasm)
   perf/compiler/T12150.run             T12150 [stat not good enough] (optasm)
   perf/should_run/T14052.run           T14052 [stat not good enough] (ghci)
   perf/should_run/T14936.run           T14936 [stat too good] (normal)
   perf/compiler/MultiLayerModules.run  MultiLayerModules [stat not good enough] (normal)

Framework failures:
   plugins/plugins07.run                             plugins07 [normal] (pre_cmd failed: 2)
   plugins/T10420.run                                T10420 [normal] (pre_cmd failed: 2)
   plugins/T11244.run                                T11244 [normal] (pre_cmd failed: 2)
   safeHaskell/check/pkg01/ImpSafeOnly08.run         ImpSafeOnly08 [runTest] (Unhandled exception during cleanup: Unable to remove folder '/c/Users/simonpj/AppData/Local/Temp/ghctest-4kcp39ag/test   spaces/./safeHaskell/check/pkg01/ImpSafeOnly08.run': [Errno 13] Permission denied: '/c/Users/simonpj/AppData/Local/Temp/ghctest-4kcp39ag/test   spaces/./safeHaskell/check/pkg01/ImpSafeOnly08.run/pdb.ImpSafeOnly08/install/x86_64-windows-ghc-8.5.20180420/safePkg01-1.0-LQOvw3iv9WM6fIcv695PaM'
Unable to start current test.)
   ../../libraries/process/tests/T9775/T9775_fail.run  T9775_fail [normal] (pre_cmd failed: 2)
   ../../libraries/process/tests/T9775/T9775_good.run  T9775_good [normal] (pre_cmd failed: 2)

Framework warnings:
   .  T13701 [numfield-no-expected] (No expected value found for bytes allocated in num_field check)

That's an uncomfortably long list.