Support for Back when bdccd3c1

 (test-lib: allow negation of
prerequisites, 2012-11-14) introduced negated predicates
(e.g. "!MINGW,!CYGWIN"), we already had 5 test files that use
NOT_MINGW (and a few MINGW) as prerequisites.

Let's not add NOT_FOO and rewrite existing ones as !FOO for both
MINGW and CYGWIN.

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