tests: turn off git-daemon tests if FIFOs are not available

The Git daemon tests create a FIFO first thing and will hang if said
FIFO is not available.

This is a problem with Git for Windows, where `mkfifo` is an MSYS2
program that leverages MSYS2's POSIX emulation layer, but
`git-daemon.exe` is a MINGW program that has not the first clue about
that POSIX emulation layer and therefore blinks twice when it sees
MSYS2's emulated FIFOs and then just stares into space.

This lets t5570-git-daemon.sh and t5811-proto-disable-git.sh pass.

Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-git-daemon.sh

@@ -23,6 +23,11 @@ then
 	test_done
 fi
 
+if test_have_prereq !PIPE
+then
+	test_skip_or_die $GIT_TEST_GIT_DAEMON "file system does not support FIFOs"
+fi
+
 LIB_GIT_DAEMON_PORT=${LIB_GIT_DAEMON_PORT-${this_test#t}}
 
 GIT_DAEMON_PID=