[tsl] Rewrite `TF_ASSERT_OK` and `TF_EXPECT_OK` in terms of `IsOk()` matcher
[tsl] Rewrite `TF_ASSERT_OK` and `TF_EXPECT_OK` in terms of `IsOk()` matcher

* This will allow using these macros on `absl::StatusOr` objects
  * Since `absl::StatusOr` lacks an `operator==` with `absl::Status`, it couldn't be used here before
  * Now it uses `tsl::testing::IsOk()` which works for both `absl::Status` and `absl::StatusOr`
* This should also likely result in nicer error messages in case of failure
