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/README

@@ -523,6 +523,11 @@ use these, and "test_set_prereq" for how to define your own.
    The filesystem we're on supports symbolic links. E.g. a FAT
    filesystem doesn't support these. See 704a3143 for details.
 
+ - SANITY
+
+   Test is not run by root user, and an attempt to write to an
+   unwritable file is expected to fail correctly.
+
 Tips for Writing Tests
 ----------------------