T5866 fails spuriously

I have seen T5866 fail on Windows here with:

--- "C:/GitLabRunner/builds/ux46CNax/0/ghc/ghc/tmp/ghctest-3z5t1nrm/test   spaces/testsuite/tests/concurrent/should_run/T5866.run/T5866.stderr.normalised"	2020-11-24 13:24:19.913012500 +0000
+++ "C:/GitLabRunner/builds/ux46CNax/0/ghc/ghc/tmp/ghctest-3z5t1nrm/test   spaces/testsuite/tests/concurrent/should_run/T5866.run/T5866.run.stderr.normalised"	2020-11-24 13:24:19.913012500 +0000
@@ -1 +0,0 @@
-T5866: thread blocked indefinitely in an STM transaction

I'm reasonably confident that this patch is not at fault.

Edited Dec 20, 2020 by Ben Gamari

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