test: validate prerequistes syntax

test: validate prerequistes syntax

Brian Carson noticed that a test piece in t5601 had a pair of single
quotes in the body, which made it into 4 parameter call to
test_expect_success, as if its test title were a prerequisite.

As the prerequisites have a specific syntax (i.e. comma separated
tokens spelled in capital letters, possibly prefixed with ! for
negation), validate them to catch such a mistake in the future.

Signed-off-by: Junio C Hamano <gitster@pobox.com>