...and various other minor cleanup:

* Moved "FLUTTER_STORAGE_BASE_URL" into a constant throughout the code. There are other strings that we should do that to but this one was relevant to the code I was changing.

* Fixed the logger's handling of slow warnings. Previously it deleted too much text. Fixed the test for that to actually verify it entirely, too.

* Made the logger delete the slow warning when it's finished.

* Fixed 'Please choose one (To quit, press "q/Q")' message to be the cleaner 'Please choose one (or "q" to quit)'.

* Added a debug toString to ValidationResult for debugging purposes (not used).

* In http_host_validator:

  - Shortened constant names to be clearer (e.g. kPubDevHttpHost -> kPubDev).
  - Added GitHub as a tested host since when you run `flutter` we hit that immediately.
  - Renamed the check "Network resources".
  - Updated the `slowWarning` of the check to say which hosts are pending.
  - Removed all timeout logic. Timeouts violate our style guide.
  - Removed `int.parse(... ?? '10')`; passing a constant to `int.parse` is inefficient.
  - Replaced the `_HostValidationResult` class with `String?` for simplicity.
  - Improved the error messages to be more detailed.
  - Removed all checks that dependened on the stringification of exceptions. That's very brittle.
  - Added a warning specifically for HandshakeException that talks about the implications (MITM attacks).
  - Replaced exception-message-parsing logic with just calling `Uri.tryParse` and validating the result.
  - Replaced a lot of list-filtering logic with just a single for loop to check the results.
  - Replaced code that added a constant to a known-empty list with just returning a constant list.
  - Revamped the logic for deciding which hosts to check to just use a single chain of if/else blocks instead of getters, lists literals with `if` expressions, `??`, functions, etc spread over multiple places in the code.