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

Skip a test added in f1e3df3 (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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

t/t4202-log.sh

@@ -1992,10 +1992,13 @@ test_expect_success GPG 'log --show-signature for merged tag with GPG failure' '
 	git tag -s -m signed_tag_msg signed_tag_fail &&
 	git checkout plain-fail &&
 	git merge --no-ff -m msg signed_tag_fail &&
-	TMPDIR="$(pwd)/bogus" git log --show-signature -n1 plain-fail >actual &&
-	grep "^merged tag" actual &&
-	grep "^No signature" actual &&
-	! grep "^gpg: Signature made" actual
+	if ! test_have_prereq VALGRIND
+	then
+		TMPDIR="$(pwd)/bogus" git log --show-signature -n1 plain-fail >actual &&
+		grep "^merged tag" actual &&
+		grep "^No signature" actual &&
+		! grep "^gpg: Signature made" actual
+	fi
 '
 
 test_expect_success GPGSM 'log --graph --show-signature for merged tag x509' '