t0051: use "skip_all" under !MINGW in single-test file

t0051: use "skip_all" under !MINGW in single-test file

Have this file added in 06ba9d03e34 (t0051: test GIT_TRACE to a
windows named pipe, 2018-09-11) use the same "skip_all" pattern as an
existing Windows-only test added in 0e218f91c29 (mingw: unset PERL5LIB
by default, 2018-10-30) uses.

This way TAP consumers like "prove" will show a nice summary when the
test is skipped. Instead of:

    $ prove t0051-windows-named-pipe.sh
    [...]
    t0051-windows-named-pipe.sh .. ok
    [...]

We will prominently show a "skipped" notice:

    $ prove t0051-windows-named-pipe.sh
    [...]
    t0051-windows-named-pipe.sh ... skipped: skipping Windows-specific tests
    [...]

This is because we are now making use of the right TAP-y way to
communicate this to the consumer. I.e. skipping the whole test file,
v.s. skipping individual tests (in this case there's only one test).

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