v.io/x/devtools/jiri-test: improved error handling for commandline args.
v.io/x/devtools/jiri-test: improved error handling for commandline args.

Check for the existence of all tests before running any tests so that
errors of the form 'test run vanadium-go-test --pkgs=x' produce
an error immediately rather than after running vanadium-go-test.
This was prompted by v.io/i/1056

Change-Id: I2fbbd6c73eabd93619d4dbff0a7844c8c3dcaace

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools