t7610: test for mktemp before test execution

t7610: test for mktemp before test execution

mktemp is not available on all platforms, so the test
'temporary filenames are used with mergetool.writeToTemp'
fails there.
This patch does not replace mktemp but just disables
the test that otherwise would fail.
mergetool checks itself before executing mktemp and
reports an error.

Signed-off-by: Armin Kunaschik <megabreit@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>