Document the unittest module in manpages

This adds the manual pages: shtk_unittest_add_fixture(3),
shtk_unittest_add_test(3), shtk_unittest_assert_command(3),
shtk_unittest_assert_equal(3), shtk_unittest_assert_not_equal(3),
shtk_unittest_delayed_fail(3), shtk_unittest_expect_command(3),
shtk_unittest_expect_equal(3), shtk_unittest_expect_not_equal(3),
shtk_unittest_fail(3), shtk_unittest_main(3),
shtk_unittest_set_expected_failure(3) and shtk_unittest_skip(3).