[ddc] Add test abstractions to hot reload suite

Provides a representation for all information needed to perform the
diff checks and run the tests. Uses classes as more structured
representation to ease an upcoming refactor where the entire suite
is not defined in the main method.

Only read and iterate and parse the file names a single time and collect
all needed information at this time.

Fixes `--debug` command line flag that was accidentally broken in
https://dart-review.googlesource.com/c/sdk/+/391686.

Change-Id: Ifb6005d85d79e1edb66473b3963cb800dc5e8569
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/392404
Reviewed-by: Mark Zhou <markzipan@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>