Bug 1023935 - Use 'mozrunner-startup' instead of 'automation' as the test name during startup; r=ahal

Until we run the first test, we set self.last_test to a dummy value,
so if we time out, we'll end up with a vaguely understandable error:
TEST-UNEXPECTED-FAIL | <test_name> | application timed out after 330.0 seconds with no output

This makes the test name slightly more unique, so as to make TBPL's
matching against bugs less prone to false positives.