`test` success when all tests were filtered out

[grosser]

What is the underlying problem you're trying to solve?

opa test file.rego --run whoops

Describe the ideal solution

either:

• fails when all tests were filtered out
• add an option for --fail-on-empty

Additional Context

for humans that might be obvious, but when either outputting coverage or json or using in some other automated way it is not obvious that no tests were run

[anderseknert]

Agreed, running opa test in an empty directory or where all tests have been filtered out... that's almost certainly not what the user wanted. I'm not sure we can change that default at this point (but maybe I'm too cautious?), so the --fail-on-empty option sounds good to me.

[srenatus]

Go would print a warning and PASS in this case.

[grosser]

checked rspec (ruby test tool) and it also silently fails on empty dir or example that does not match anything
so opt-in sounds like the way to go

[grosser]

#7960