Clean up the test runner's output reporting.

- Get rid of the redundant failure summary which no one seems to like.
- Remove "diff" mode since we don't use status files any more. Its
  job was to print the results in the form of a status file diff.
- Add "--report-failures" to print the list of failing tests at the end
  of a run.
- Clean up newline handling around the compact
  "[00:01 | 100% | +   33 | -    3]" output lines. Since those don't
  print a trailing newline, it would effectively eat the preceding
  newline of other output.
- Clean up some other code while I was in there.

Change-Id: Iaa5e1e196892e653bbc5eca5eebf0f8f3c4f3ae0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/143101
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>