Improve test suite to handle external servers better. (#9033)

This commit revives the improves the ability to run the test suite against
external servers, instead of launching and managing `redis-server` processes as
part of the test fixture.

This capability existed in the past, using the `--host` and `--port` options.
However, it was quite limited and mostly useful when running a specific tests.
Attempting to run larger chunks of the test suite experienced many issues:

* Many tests depend on being able to start and control `redis-server` themselves,
and there's no clear distinction between external server compatible and other
tests.
* Cluster mode is not supported (resulting with `CROSSSLOT` errors).

This PR cleans up many things and makes it possible to run the entire test suite
against an external server. It also provides more fine grained controls to
handle cases where the external server supports a subset of the Redis commands,
limited number of databases, cluster mode, etc.

The tests directory now contains a `README.md` file that describes how this
works.

This commit also includes additional cleanups and fixes:

* Tests can now be tagged.
* Tag-based selection is now unified across `start_server`, `tags` and `test`.
* More information is provided about skipped or ignored tests.
* Repeated patterns in tests have been extracted to common procedures, both at a
  global level and on a per-test file basis.
* Cleaned up some cases where test setup was based on a previous test executing
  (a major anti-pattern that repeats itself in many places).
* Cleaned up some cases where test teardown was not part of a test (in the
  future we should have dedicated teardown code that executes even when tests
  fail).
* Fixed some tests that were flaky running on external servers.

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