Tests (MinGW): Disable mkfifo-based tests

With MSys2, there is actually an implementation of mkfifo available. The
only problem is that it is only emulating named pipes through the MSys2
runtime; The Win32 API has no idea about named pipes, hence the Git
executable cannot access those pipes either.

The symptom is that Git fails with a '<name>: No such file or directory'
because MSys2 emulates named pipes through special-crafted '.lnk' files.

The solution is to tell the test suite explicitly that we cannot use
named pipes when we want to test a MinGW Git.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/test-lib.sh

@@ -998,7 +998,7 @@ test_i18ngrep () {
 test_lazy_prereq PIPE '
 	# test whether the filesystem supports FIFOs
 	case $(uname -s) in
-	CYGWIN*)
+	CYGWIN*|MINGW*)
 		false
 		;;
 	*)