swift234

[SR-237] Move SKIP_TEST_* arguments to Python

- Migrate `SKIP_TEST_IOS`, `SKIP_TEST_TVOS`, and `SKIP_TEST_WATCHOS` to
  Python.
- In the `build-script-impl` shellscript, only deal with
  `SKIP_TEST_*_HOST` and `SKIP_TEST_*_SIMULATOR` variables.
- Introduce a `--host-test` flag to the Python `build-script` in order
  to allow users to specify whether to run host tests. These flags
  still don't do anything.
- Fix typo: `skip-build-tvos_device` was meant to be `skip-build-tvos-device`.