[analysis_server] Make SemanticTokens tests more readable with dot shorthands

These tests have always been a bit difficult to read (and review) due to wrapping of expectations. This switches them all to dot shorthands to reduce them to one line.

To do this, because we have both standard + custom classes, I added some private enums that just include the values from both the standard + custom sets. While these need maintaining as we add more, I think the overall readability improvements makes it worthwhile. If we get some kind of static extensions in future, we might be able to avoid this.

I also remove the duplicate "label" type since it's now in the standard set and not required in our custom set.

Change-Id: Icc47b95766fe9b1d292b2a4e9889b3d005c62cee
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/542300
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>