test-lib: Add support for multiple test prerequisites

Change the test_have_prereq function in test-lib.sh to support a
comma-separated list of prerequisites. This is useful for tests that
need e.g. both POSIXPERM and SANITY.

The implementation was stolen from Junio C Hamano and Johannes Sixt,
the tests and documentation were not. See the "Tests in Cygwin" thread
in May 2009 for the originals:

http://thread.gmane.org/gmane.comp.version-control.git/116729/focus=118385
http://thread.gmane.org/gmane.comp.version-control.git/116729/focus=118434

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