[dartdev] add help text for 'dartdev test'
[dartdev] add help text for 'dartdev test'

Change-Id: Ibb8cc8a4995008e730f9cc4499479ca689d9edd4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/140765
Reviewed-by: Jaime Wren <cd1ab83d11dd618c6b8cd01592020d0bd5183fdc@google.com>
Commit-Queue: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk