Update tools/regression_tester

* Clarify wording (don't use 'success' and 'failure' instead use 'found'
  and 'no regression test')
* If don't pass gerrit review number run on current branch.  In
  preparation for running via jenkins.
* If no tests were changed, don't run any tests

Change-Id: I5e876e4e5071fc27461d2411fc33db1bf1724405
Merge "Update tools/regression_tester"