Add simplified unittest replacement to package:expect.
(It is used in multi-tests, so it needs to be in an accessible place without using ".." in its path).
Change-Id: I60aa212ed67ffbdda17ca18fce1cdff16237f4fe
Reviewed-on: https://dart-review.googlesource.com/65700
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>
Reviewed-by: Jenny Messerly <jmesserly@google.com>