mingw: disable t9020

POSIX-to-Windows path mangling would make it fail. Symptoms:

	++ init_git
	++ rm -fr .git
	++ git init
	Initialized empty Git repository in [...]
	++ git remote add svnsim testsvn::sim:///usr/src/git/wip5/t/t9154/svn.dump
	++ git remote add svnfile testsvn::file:///usr/src/git/wip5/t/t9154/svn.dump
	++ git fetch svnsim
	progress Imported commit 1.
	fatal: Write to frontend failed: Bad file descriptor
	fast-import: dumping crash report to .git/fast_import_crash_23356
	fatal: error while running fast-import
	fatal: unexpected end of fast-import feedback
	error: last command exited with $?=128
	not ok 1 - simple fetch

Since the remote-svn project seems to be dormant at the moment (and not
complete enough to be used, which is a pity), let's just skip this test
on Windows.

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

t/t9020-remote-svn.sh

@@ -12,6 +12,12 @@ then
 	test_done
 fi
 
+if test_have_prereq MINGW
+then
+	skip_all='skipping remote-svn tests for lack of POSIX'
+	test_done
+fi
+
 # Override svnrdump with our simulator
 PATH="$HOME:$PATH"
 export PATH PYTHON_PATH GIT_BUILD_DIR