t: annotate !PTHREADS tests with !FAIL_PREREQS

t: annotate !PTHREADS tests with !FAIL_PREREQS

Some tests in t5300 and t7810 expect us to complain about a "--threads"
argument when Git is compiled without pthread support. Running these
under GIT_TEST_FAIL_PREREQS produces a confusing failure: we pretend to
the tests that there is no pthread support, so they expect the warning,
but of course the actual build is perfectly happy to respect the
--threads argument.

We never noticed before the recent a926c4b904 (tests: remove most uses
of C_LOCALE_OUTPUT, 2021-02-11), because the tests also were marked as
requiring the C_LOCALE_OUTPUT prerequisite. Which means they'd never
have run in FAIL_PREREQS mode, since it would always pretend that the
locale prereq was not satisfied.

These tests can't possibly work in this mode; it is a mismatch between
what the tests expect and what the build was told to do. So let's just
mark them to be skipped, using the special prereq introduced by
dfe1a17df9 (tests: add a special setup where prerequisites fail,
2019-05-13).

Reported-by: Son Luong Ngoc <sluongng@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>