Test: catch if trash cannot be removed

When your test creates an unwritable directory that test framework cannot
clean out by "rm -fr trash", later tests cannot start in a fresh state
they expect to.  Detect this and error out early.

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

t/test-lib.sh

@@ -396,7 +396,12 @@ fi
 
 # Test repository
 test=trash
-rm -fr "$test"
+rm -fr "$test" || {
+	trap - exit
+	echo >&5 "FATAL: Cannot prepare test area"
+	exit 1
+}
+
 test_create_repo $test
 cd "$test"