Use `TEST_WS` in all remote-externalities tests (#3284)

Refactor in accordance with
https://github.com/paritytech/polkadot-sdk/issues/2245#issuecomment-1937025951

Prior to this PR, the `remote_tests` test module would either use
`TEST_WS` or `DEFAULT_HTTP_ENDPOINT`.

With the PR, `TEST_WS` is the default for the `remote_tests` test module
and the fallback is `DEFAULT_HTTP_ENDPOINT`.

The only downside I see to this PR is that for particular tests in the
`remote_tests` module, one would want to use a different http endpoint.
If that is the case, they would have to manually hardcode the http
endpoint for that particular test.

Note: The `TEST_WS` node should fulfill the role for all test cases e.g.
include child tries.

Give it a _try_:
```
TEST_WS=wss://rococo-try-runtime-node.parity-chains.parity.io:443

 cargo test --features=remote-test -p frame-remote-externalities -- --nocapture
```

---------

Co-authored-by: Oliver Tale-Yazdi <[email protected]>