branches: add tests for `git_branch_is_checked_out`

We currently do not have any tests at all for the
`git_branch_is_checked_out` function. Add some basic ones.

tests/refs/branches/checkedout.c

+#include "clar_libgit2.h"
+#include "refs.h"
+#include "worktree/worktree_helpers.h"
+
+static git_repository *repo;
+
+static void assert_checked_out(git_repository *repo, const char *branch, int checked_out)
+{
+	git_reference *ref;
+
+	cl_git_pass(git_reference_lookup(&ref, repo, branch));
+	cl_assert(git_branch_is_checked_out(ref) == checked_out);
+
+	git_reference_free(ref);
+}
+
+void test_refs_branches_checkedout__simple_repo(void)
+{
+	repo = cl_git_sandbox_init("testrepo");
+	assert_checked_out(repo, "refs/heads/master", 1);
+	assert_checked_out(repo, "refs/heads/executable", 0);
+	cl_git_sandbox_cleanup();
+}
+
+void test_refs_branches_checkedout__worktree(void)
+{
+	static worktree_fixture fixture =
+	    WORKTREE_FIXTURE_INIT("testrepo", "testrepo-worktree");
+
+	setup_fixture_worktree(&fixture);
+
+	assert_checked_out(fixture.repo, "refs/heads/master", 1);
+	assert_checked_out(fixture.repo, "refs/heads/testrepo-worktree", 1);
+
+	assert_checked_out(fixture.worktree, "refs/heads/master", 1);
+	assert_checked_out(fixture.worktree, "refs/heads/testrepo-worktree", 1);
+
+	cleanup_fixture_worktree(&fixture);
+}