T7170 fails on Windows

As of https://gitlab.haskell.org/ghc/ghc/-/jobs/97843.

--- ffi/should_run/T7170.run/T7170.stderr.normalised	2019-06-11 06:44:45.850716500 +0000
+++ ffi/should_run/T7170.run/T7170.run.stderr.normalised	2019-06-11 06:44:45.852670300 +0000
@@ -1 +0,0 @@
-T7170: thread blocked indefinitely in an MVar operation

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