A few minor test improvements

This patch adds a few niceties to the test suite:

- allows matching test groups case insensitive and emit warnings when
  we give test group names that don't match anything
- add a new operator that is similar to assertEqual in Python: it
  tests for equality and emits the two values in case of error

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>