test-lib: allow negation of prerequisites

You can set and test a prerequisite like this: test_set_prereq FOO test_have_prereq FOO && echo yes You can negate the test in the shell like this: ! test_have_prereq && echo no However, when you are using the automatic prerequisite checking in test_expect_*, there is no opportunity to use the shell negation. This patch introduces the syntax "!FOO" to indicate that the test should only run if a prerequisite is not meant. One alternative is to set an explicit negative prerequisite, like: if system_has_foo; then test_set_prereq FOO else test_set_prereq NO_FOO fi However, this doesn't work for lazy prerequisites, which associate a single test with a single name. We could teach the lazy prereq evaluator to set both forms, but the code change ends up quite similar to this one (because we still need to convert NO_FOO into FOO to find the correct lazy script). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>