add test for shallow feature flag

tests/shallow/feature_flag.c

+#include "clar_libgit2.h"
+
+void test_shallow_feature_flag__set_feature_flag(void)
+{
+    cl_must_pass(git_libgit2_opts(GIT_OPT_ENABLE_SHALLOW, 1));
+}
+
+void test_shallow_feature_flag__unset_feature_flag(void)
+{
+    cl_must_pass(git_libgit2_opts(GIT_OPT_ENABLE_SHALLOW, 0));
+}