Remove runtest hard dependency on test_defs.xml files.

Previously runtest would fail hard if development/test_runner/test_defs.xml
file was not present, even if it wasn't needed (ie running in --path mode).

This commit skips test_defs parsing if --path is specified, and only parses
development/test_runner/test_defs.xml if its present.

Bug 4556556

Change-Id: Ief2dbbdab5c7e82e5db64414392d6e2a564e815d
1 file changed