### What changes were proposed in this pull request?
- Update `testthat` to >= 2.0.0
- Replace of `testthat:::run_tests` with `testthat:::test_package_dir`
- Add trivial assertions for tests, without any expectations, to avoid skipping.
- Update related docs.
### Why are the changes needed?
`testthat` version has been frozen by [SPARK-22817](https://issues.apache.org/jira/browse/SPARK-22817) / https://github.com/apache/spark/pull/20003, but 1.0.2 is pretty old, and we shouldn't keep things in this state forever.
### Does this PR introduce any user-facing change?
No.
### How was this patch tested?
- Existing CI pipeline:
- Windows build on AppVeyor, R 3.6.2, testthtat 2.3.1
- Linux build on Jenkins, R 3.1.x, testthat 1.0.2
- Additional builds with thesthat 2.3.1 using [sparkr-build-sandbox](https://github.com/zero323/sparkr-build-sandbox) on c7ed64af9e697b3619779857dd820832176b3be3
R 3.4.4 (image digest ec9032f8cf98)
```
docker pull zero323/sparkr-build-sandbox:3.4.4
docker run zero323/sparkr-build-sandbox:3.4.4 zero323 --branch SPARK-23435 --commit c7ed64af9e697b3619779857dd820832176b3be3 --public-key https://keybase.io/zero323/pgp_keys.asc
```
3.5.3 (image digest 0b1759ee4d1d)
```
docker pull zero323/sparkr-build-sandbox:3.5.3
docker run zero323/sparkr-build-sandbox:3.5.3 zero323 --branch SPARK-23435 --commit
c7ed64af9e697b3619779857dd820832176b3be3 --public-key https://keybase.io/zero323/pgp_keys.asc
```
and 3.6.2 (image digest 6594c8ceb72f)
```
docker pull zero323/sparkr-build-sandbox:3.6.2
docker run zero323/sparkr-build-sandbox:3.6.2 zero323 --branch SPARK-23435 --commit c7ed64af9e697b3619779857dd820832176b3be3 --public-key https://keybase.io/zero323/pgp_keys.asc
````
Corresponding [asciicast](https://asciinema.org/) are available as 10.5281/zenodo.3629431
[](https://doi.org/10.5281/zenodo.3629431)
(a bit to large to burden asciinema.org, but can run locally via `asciinema play`).
----------------------------
Continued from #27328Closes#27359 from zero323/SPARK-23435.
Authored-by: zero323 <mszymkiewicz@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
298d0a5102
