t0070 "mktemp to unwritable directory" needs SANITY

Use the SANITY prerequisite when testing if a temp file can
be created in a read only directory.
Skip the test under CYGWIN, or skip it under Unix/Linux when
it is run as root.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0070-fundamental.sh

@@ -17,7 +17,7 @@ test_expect_success 'mktemp to nonexistent directory prints filename' '
 	grep "doesnotexist/test" err
 '
 
-test_expect_success POSIXPERM 'mktemp to unwritable directory prints filename' '
+test_expect_success POSIXPERM,SANITY 'mktemp to unwritable directory prints filename' '
 	mkdir cannotwrite &&
 	chmod -w cannotwrite &&
 	test_when_finished "chmod +w cannotwrite" &&