test-lib: add an EXPENSIVE_ON_WINDOWS prerequisite

test-lib: add an EXPENSIVE_ON_WINDOWS prerequisite

Add an EXPENSIVE_ON_WINDOWS prerequisite to mark those tests which are
very expensive to run on Windows, but cheap elsewhere.

Certain tests that heavily stress the filesystem or run a lot of shell
commands are disproportionately expensive on Windows, this
prerequisite will later be used by a tests that runs in 4-8 seconds on
a modern Linux system, but takes almost 10 minutes on Windows.

There's no reason to skip such tests by default on other platforms,
but Windows users shouldn't need to wait around while they finish.

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