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>

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