t/clar: fix compatibility with NonStop

The NonStop platform does not have `mkdtemp()` available, which we rely
on in `build_sandbox_path()`. Fix this issue by using `mktemp()` and
`mkdir()` instead on this platform.

This has been cherry-picked from the upstream pull request at [1].

[1]: clar-test/clar#96

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/unit-tests/clar/clar/sandbox.h

@@ -120,6 +120,12 @@ static int build_sandbox_path(void)
 	if (_mktemp(_clar_path) == NULL)
 		return -1;
 
+	if (mkdir(_clar_path, 0700) != 0)
+		return -1;
+#elif defined(__TANDEM)
+	if (mktemp(_clar_path) == NULL)
+		return -1;
+
 	if (mkdir(_clar_path, 0700) != 0)
 		return -1;
 #elif defined(_WIN32)