log test: skip a failing mkstemp() test under valgrind

log test: skip a failing mkstemp() test under valgrind

Skip a test added in f1e3df31699 (t: increase test coverage of
signature verification output, 2020-03-04) when running under
valgrind. Due to valgrind's interception of mkstemp() this test will
fail with:

	+ pwd
	+ TMPDIR=[...]/t/trash directory.t4202-log/bogus git log --show-signature -n1 plain-fail
	==7696== VG_(mkstemp): failed to create temp file: [...]/t/trash directory.t4202-log/bogus/valgrind_proc_7696_cmdline_d545ddcf
	[... 10 more similar lines omitted ..]
	valgrind: Startup or configuration error:
	valgrind:    Can't create client cmdline file in [...]/t/trash directory.t4202-log/bogus/valgrind_proc_7696_cmdline_6e542d1d
	valgrind: Unable to start up properly.  Giving up.
	error: last command exited with $?=1

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