Iterate on new test icons experiments

[luabud]

These experiments enable icons for "run all failed tests" and "refresh tests" in the test explorer. We have a hypothesis that these are useful actions to have in the test explorer, so with the experiments we will be able see if it holds true with all kinds of users.

• Turn "run all failed tests" to 100% of insider and 10% of stable users
• Turn "run all failed tests" to 50% of stable users (provided we see good results)
• Turn "run all failed tests" to 100% of stable users (provided we see good results)
• Turn "refresh tests" to 100% of insiders and 50% of stable users
• Turn "refresh tests" to 100% of stable users (provided we see good results)

[luabud]

To verify just clone https://github.com/luabud/sample_project, open a Python file and open the test explorer:

[aeschli]

I verified that I see the two actions in the tests view. Please reopen if there's more to verify.

Python v2021.9.1246542782, with insiders (daily)