Bug in Google Cloud SDK support for Firebase Test Lab

[Conor O'Donnell]

When reading an arg-file for a Firebase Test Lab run, the "test-targets" field will cause an error if it is a list of strings, like the docs suggest it can be.

This is due to it not having an entry in the _FILE_ARG_VALIDATORS map in lib/googlecloudsdk/api_lib/test/arg_validate.py.

By adding the line 

'test_targets': ValidateStringList,

to that map, the argument will be read properly and the test run will proceed as normal.

[Tony Kai Jie Ren]

We have notified our developers of this issue.

Thank you for the report!