[lldb][docs] Document how to test specific layers (#205581)
This is motivated by the fact that we have the ability to test almost
any component of the debug session on its own, but it's hard to find
those tests.
If we put AI aside, you can't look for "test that lldb doesn't fault
when qProcessInfo
contains foo". Even though that is a thing we can test.
So in this change I'm adding a section to the testing docs with some
starting points that
people can search for.
It will be incomplete but we can add to it over time.
I will need someone to write the DAP part in a follow up PR, as I'm not
familiar with the layers there.