[package:testing] Various updates

 * Delete unused stuff
 * rename 'path' to 'root'
 * accept 'includeEndsWith' as a plain text string so we often can avoid
   using regexp
 * accept 'subRoots' to filter to directories faster and more precisly
   than when using regexps in 'pattern'
 * make 'list' async instead of async* (no more yield stuff which we
   promptly turn into a list when actually using it)

Note that some changes in testing.json is not 100% semantic-preserving,
e.g. the "parser_all" suite previously had a pattern "/tests/.*\\.dart$"
which was probably meant to include all dart files in the "tests" folder
in the root, but in fact included all dart files in a "tests" folder
anywhere. The updated version does not.

Change-Id: Idd014274f86bf6214dee0753a7738ec80bc6a49e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/358442
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>