tests: add a special setup where prerequisites fail

As discussed in [1] there's a regression in the "pu" branch now
because a new test implicitly assumed that a previous test guarded by
a prerequisite had been run. Add a "GIT_TEST_FAIL_PREREQS" special
test setup where we'll skip (nearly) all tests guarded by
prerequisites, allowing us to easily emulate those platform where we
don't run these tests.

As noted in the documentation I'm adding I'm whitelisting the SYMLINKS
prerequisite for now. A lot of tests started failing if we lied about
not supporting symlinks. It's also unlikely that we'll have a failing
test due to a hard dependency on symlinks without that being the
obvious cause, so for now it's not worth the effort to make it work.

1. https://public-inbox.org/git/nycvar.QRO.7.76.6.1905131531000.44@tvgsbejvaqbjf.bet/

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

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