Improve handling of SKIPPED_TESTS in e2e-cp.sh

[kyrtapz]

Improve how we handle skipped tests in e2e-cp.sh.
Currently the SKIPPED_TESTS is a variable that we add "|test" to, this is error prone and can cause us skipping tests unintentionally.
Example bug: #4227

[tssurya]

another example: #4253

[jluhrsen]

@kyrtapz , @tssurya , do you have any specific ideas or examples to do this better? I'd like to work on this if possible. I can dig in and come up with my own idea if you don't have anything off the top of your head.

[tssurya]

@jluhrsen : go wild and reinvent this :) I don't have any specific preference but can we see how K8s/K8s does these things just to get some motivation maybe?