[Feature] Easy way of checking output to stderr or stdout

[chipzoller]

What would you like to be added:

An easier method of checking output result of a command/manifest than writing a custom script.

Why is this needed:

Checking the output of a command, especially when testing admission controllers, is very common. Warnings can be written to stdout and errors such as validation failures can be written to stderr. Today in kuttl, checking the contents of this output requires a command or script. Since this is a pretty common use case, it would be nice to enable this in a simple way like combining a resource with an assert to parse the output.