Nondeterministic T3424 timeouts on CI

The T3424 test case occasionally times out on CI, forcing me to restart builds to get a green build status. Here is one example from a validate-x86_64-linux-deb9-unreg run:

Timeout happened...killed process "cd "rts/T3424.run" &&  "/builds/ghc/ghc/inplace/bin/ghc-stage2" -o T3424 T3424.hs -dcore-lint -dstg-lint -dcmm-lint -no-user-package-db -rtsopts -optc-fno-builtin -fno-warn-missed-specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -Werror=compat -dno-debug-output  "...

Compile failed (exit code 99) errors were:
[1 of 1] Compiling Main             ( T3424.hs, T3424.o )

*** unexpected failure for T3424(normal)

<elided>

Unexpected results from:
TEST="T3424"

SUMMARY for test run started at Thu Feb 21 05:18:04 2019 UTC
 1:15:50 spent to go through
    6839 total tests, which gave rise to
   21103 test cases, of which
   13986 were skipped

      38 had missing libraries
    6977 expected passes
     101 expected failures

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

Unexpected failures:
   rts/T3424.run  T3424 [exit code non-0] (normal)