[analysis_server] Move all EditableArgument tests to a shared mixin a…

…nd run for legacy

This moves all tests (without any changes) to a shared mixin, and then applies that mixin to the original test class. It also adds a new test class for LSP-over-Legacy, meaning the tests will now run for both protocols.

There are two failing tests when using LSP-over-Legacy so they are overridden with `@FailingTest()` temporarily.

Change-Id: I8c7efcf8f5d85d874b3483f4f3953d4d8b68a31a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/404621
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>
Reviewed-by: Phil Quitslund <[email protected]>