Explicit tests

## Summary

Allow marking a test or test class to be ignored unless it is explicitly selected for running, i.e. such test(s) won't be executed when using _Run All Tests_ but will execute when selected for individual run.
Similar to NUnit's `[Explicit]` attribute: https://docs.nunit.org/articles/nunit/writing-tests/attributes/explicit.html

## Background and Motivation

Some tests are meant only to be used by developers on development box to help with debugging or other code simulations.
Currently, we mark these as `[Ignored]` and enable them only as needed. Unfortunately, this leads to many accidental situations where the test is left un-ignored and committed to the repository.

## Proposed Feature

Adding an `[Explicit]` attribute known from other test frameworks might be a good fit.
* https://github.com/xunit/xunit/issues/2518
* https://docs.nunit.org/articles/nunit/writing-tests/attributes/explicit.html

Start by reviewing the existing [Ignored] behavior and NUnit's [Explicit] semantics linked in the issue. Define how an [Explicit] attribute on a test or test class interacts with Run All Tests versus an individual test run; done means explicit tests are skipped from the former and run by the latter.

Written by the indexing model from the issue text.