Allow all runtime tests for a language to be run via a single command.
Allow all runtime tests for a language to be run via a single command.

This was intended behavior per the README, but running tests without the --test
flag caused an error. Users can now omit the --test flag to run every test for a
runtime.

PiperOrigin-RevId: 280522025

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor