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> Signed-off-by: Junio C Hamano <gitster@pobox.com>