Add an "auto" mode for dart.testInvocationMode that tries to pick the best mode automatically #5716

We have a setting to run tests by line (instead of by name), but it's opt-in.

We could solve some issues by automatically defaulting to using lines if we think that running by-name will fail. For example #5707 where the group() call is not inside main() is problematic because the caller might have wrapped us in a group() and we won't include that in the computed full name.

Similarly, we have some workarounds for dynamic test names (test('foo $bar'...) to replace variables with .*, but running by line might be a cleaner way to do this.

When checking my config, it seems I may have been using by-line for a long time not realising (and forgetting that it wasn't default) so it might be pretty solid.

Some things we should do:

• Change the default to undefined/"auto" instead of "name" so we can still force running all tests by-name in case there are any problems when we automatically use lines
• Verify behaviour with tests that come from results (like pkg:test_reflective_loader) because we don't currently track their locations and might do the wrong thing (although, if we minimize where we default to by-line for, that might minimize this)

Probably candidates to run by-line by default:

• Any variables/dynamic names (we may want to use the presence of quotes in the Outline name for this?)
• Test nodes outside of the main top level function