There is something odd afoot in the tests in testsuite/tests/plugins on Windows. In any given testsuite run at least half a dozen of these tests fail for one reason or another (often a timeout sometimes another). I'm collecting these failures here to try to make sense of them:

=====> T16260(normal) 6833 of 6839 [2, 14, 0]
cd "plugins/T16260.run" && $MAKE -s --no-print-directory -C simple-plugin package.T16260 TOP=/c/GitLabRunner/builds/78d7d3f9/0/ghc/ghc/testsuite
cd "plugins/T16260.run" && $MAKE -s --no-print-directory T16260  --- plugins/T16260.run/T16260.stdout.normalised	2019-03-06 03:20:38.902713500 +0000
+++ plugins/T16260.run/T16260.run.stdout.normalised	2019-03-06 03:20:38.903686400 +0000
@@ -1,4 +0,0 @@
-False
-None
-True
-Safe

Actual stdout output differs from expected:
diff -uw "plugins/T16260.run/T16260.stdout.normalised" "plugins/T16260.run/T16260.run.stdout.normalised"