tests: make sure nested lazy prereqs work reliably

Some test prereqs depend on other prereqs, so in a couple of cases we
have nested prereqs that look something like this:

  test_lazy_prereq FOO '
      test_have_prereq BAR &&
      check-foo
  '

This can be problematic, because lazy prereqs are evaluated in the
'$TRASH_DIRECTORY/prereq-test-dir' directory, which is the same for
every prereq, and which is automatically removed after the prereq has
been evaluated.  So if the inner prereq (BAR above) is a lazy prereq
that hasn't been evaluated yet, then after its evaluation the
'prereq-test-dir' shared with the outer prereq will be removed.
Consequently, 'check-foo' will find itself in a non-existing
directory, and won't be able to create/access any files in its cwd,
which could result in an unfulfilled outer prereq.

Luckily, this doesn't affect any of our current nested prereqs, either
because the inner prereq is not a lazy prereq (e.g. MINGW, CYGWIN or
PERL), or because the outer prereq happens to be checked without
touching any paths in its cwd (GPGSM and RFC1991 in 'lib-gpg.sh').

So to prevent nested prereqs from interfering with each other let's
evaluate each prereq in its own dedicated directory by appending the
prereq's name to the directory name, e.g. 'prereq-test-dir-SYMLINKS'.
In the test we check not only that the prereq test dir is still there,
but also that the inner prereq can't mess with the outer prereq's
files.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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