tests: submodule: fix declaration of test

The testcase "submodule::lookup::cached" was declared with a single
underscore separating the test suide and test name, only. As the clar
parser only catches tests with two underscores, it was never executed.
Add in the second underscore to actually have it detected and executed.

tests/submodule/lookup.c

@@ -389,7 +389,8 @@ void test_submodule_lookup__renamed(void)
 	cl_assert_equal_i(8, data.count);
 }
 
-void test_submodule_lookup_cached(void) {
+void test_submodule_lookup__cached(void)
+{
 	git_submodule *sm;
 	git_submodule *sm2;
 	/* See that the simple tests still pass. */