Feature Request: unittest - Support target_under_test with cfg="exec"

I would like to write an analysis test for a rule that is intended to be used for build tool targets. I.e. targets that will be depended upon with `cfg = "exec"`. However, AFAICT, the current API exposed by `analysistest.make` does not allow to define the `target_under_test` as a dependency with `cfg = "exec"`.

Notably, it does support changed settings for the target. However, I'm not aware of a way achieve the same effect as `cfg = "exec"` through this mechanism. If there is one, please do let me know.

My current work around is to simply leave the `target_under_test` as the default `cfg = "target"`, and also set any other attributes that I may want to compare the `target_under_test` with as `cfg = "target"`.

Start by locating the analysistest.make API and reading how it currently defines target_under_test and changed settings. Trace the existing analysis-test setup for configuration handling, then determine the smallest documented behavior needed for an exec-configured target_under_test. Done means analysis tests can compare a target under cfg = "exec" while preserving existing target-configuration behavior.

Written by the indexing model from the issue text.