t0000: run prereq tests inside sub-test

We test the behavior of prerequisites in t0000 by setting up fake ones
in the main test script, trying to run some tests, and then seeing if
those tests impacted the environment correctly. If they didn't, then we
write a message and manually call exit.

Instead, let's push these down into a sub-test, like many of the other
tests covering the framework itself. This has a few advantages:

  - it does not pollute the test output with mention of skipped tests
    (that we know are uninteresting -- the point of the test was to see
    that these are skipped).

  - when running in a TAP harness, we get a useful test failure message
    (whereas when the script exits early, a tool like "prove" simply
    says "Dubious, test returned 1").

  - we do not have to worry about different test environments, such as
    when GIT_TEST_FAIL_PREREQS_INTERNAL is set. Our sub-test helpers
    already give us a known environment.

  - the tests themselves are a bit easier to read, as we can just check
    the test-framework output to see what happened (and get the usual
    test_cmp diff if it failed)

A few notes on the implementation:

  - we could do one sub-test per each individual test_expect_success. I
    broke it up here into a few logical groups, as I think this makes it
    more readable

  - the original tests modified environment variables inside the test
    bodies. Instead, I've used "true" as the body of a test we expect to
    run and "false" otherwise. Technically this does not confirm that
    the body of the "true" test actually ran. We are trusting the
    framework output to believe that it truly ran, which is sufficient
    for these tests. And I think the end result is much simpler to
    follow.

  - the nested_prereq test uses a few bare "test -f" calls; I converted
    these to our usual test_path_is_* helpers while moving the code
    around.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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