T5644 failure on Windows

In https://gitlab.haskell.org/ghc/ghc/-/jobs/635501:

--- "C:/GitLabRunner/builds/1/635501/tmp/ghctest-06uy_aja/test   spaces/testsuite/tests/rts/T5644/T5644.run/T5644.stderr.normalised"	2021-03-30 15:09:04.240082900 +0000
+++ "C:/GitLabRunner/builds/1/635501/tmp/ghctest-06uy_aja/test   spaces/testsuite/tests/rts/T5644/T5644.run/T5644.run.stderr.normalised"	2021-03-30 15:09:04.240082900 +0000
@@ -1,3 +0,0 @@
-T5644: Heap exhausted;
-T5644: Current maximum heap size is 20971520 bytes (20 MB).
-T5644: Use `+RTS -M<size>' to increase it.