tests: A SANITY test prereq for testing if we're root

Some tests depend on not being able to write to files after chmod
-w. This doesn't work when running the tests as root.

Change test-lib.sh to test if this works, and if so it sets a new
SANITY test prerequisite. The tests that use this previously failed
when run under root.

There was already a test for this in t3600-rm.sh, added by Junio C
Hamano in 2283645 in 2006. That check now uses the new SANITY
prerequisite.

Some of this was resurrected from the "Tests in Cygwin" thread in May
2009:

    http://thread.gmane.org/gmane.comp.version-control.git/116729/focus=118385

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0004-unwritable.sh

@@ -15,7 +15,7 @@ test_expect_success setup '
 
 '
 
-test_expect_success POSIXPERM 'write-tree should notice unwritable repository' '
+test_expect_success POSIXPERM,SANITY 'write-tree should notice unwritable repository' '
 
 	(
 		chmod a-w .git/objects .git/objects/?? &&
@@ -27,7 +27,7 @@ test_expect_success POSIXPERM 'write-tree should notice unwritable repository' '
 
 '
 
-test_expect_success POSIXPERM 'commit should notice unwritable repository' '
+test_expect_success POSIXPERM,SANITY 'commit should notice unwritable repository' '
 
 	(
 		chmod a-w .git/objects .git/objects/?? &&
@@ -39,7 +39,7 @@ test_expect_success POSIXPERM 'commit should notice unwritable repository' '
 
 '
 
-test_expect_success POSIXPERM 'update-index should notice unwritable repository' '
+test_expect_success POSIXPERM,SANITY 'update-index should notice unwritable repository' '
 
 	(
 		echo 6O >file &&
@@ -52,7 +52,7 @@ test_expect_success POSIXPERM 'update-index should notice unwritable repository'
 
 '
 
-test_expect_success POSIXPERM 'add should notice unwritable repository' '
+test_expect_success POSIXPERM,SANITY 'add should notice unwritable repository' '
 
 	(
 		echo b >file &&