repo: free data in the discovery tests

Shocked that our leak checkers didn't find this earlier.

tests/libgit2/repo/discover.c

@@ -122,7 +122,10 @@ void test_repo_discover__cleanup(void)
 void test_repo_discover__discovering_repo_with_exact_path_succeeds(void)
 {
 	cl_git_pass(git_repository_discover(&discovered, DISCOVER_FOLDER, 0, ceiling_dirs.ptr));
+	git_buf_dispose(&discovered);
+
 	cl_git_pass(git_repository_discover(&discovered, SUB_REPOSITORY_FOLDER, 0, ceiling_dirs.ptr));
+	git_buf_dispose(&discovered);
 }
 
 void test_repo_discover__discovering_nonexistent_dir_fails(void)