mingw: disable mkfifo-based tests

MSYS2 (the POSIX emulation layer used by Git for Windows' Bash) actually
has a working mkfifo. 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 on Windows.

This lets t4056-diff-order.sh, t9010-svn-fe.sh and t9300-fast-import.sh
pass.

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

t/test-lib.sh

@@ -1000,7 +1000,7 @@ test_i18ngrep () {
 test_lazy_prereq PIPE '
 	# test whether the filesystem supports FIFOs
 	case $(uname -s) in
-	CYGWIN*)
+	CYGWIN*|MINGW*)
 		false
 		;;
 	*)