tests: iterator::workdir: fix reference count in stale test

The test `iterator::workdir::filesystem_gunk` is usually not executed,
as it is guarded by the environment variable "GITTEST_INVASIVE_SPEED"
due to its effects on speed. As such, it has become stale and does not
account for new references which have meanwhile been added to the
testrepo, causing it to fail. Fix this by raising the number of expected
references to 15.

(cherry picked from commit b8c14499)

tests/iterator/workdir.c

@@ -662,7 +662,7 @@ void test_iterator_workdir__filesystem_gunk(void)
 	/* should only have 13 items, since we're not asking for trees to be
 	 * returned.  the goal of this test is simply to not crash.
 	 */
-	expect_iterator_items(i, 13, NULL, 13, NULL);
+	expect_iterator_items(i, 15, NULL, 15, NULL);
 	git_iterator_free(i);
 	git_buf_free(&parent);
 }