Separate bevy tests from the example on how to run tests

[Weibye]

What problem does this solve or what need does it fill?

The tests currently within ./tests are also listed in the ./examples/README.md and the Cargo.toml as the example on how to test systems.

• The fact that an example does not reside within ./examples is currently blocking [Merged by Bors] - CI - Check that examples are listed in README and Cargo #1650
• I would also argue that it reduces cohesion / clarity as it is an exception from an otherwise consistent rule: That examples are always found within ./examples

What solution would you like?

Assuming ./tests will become a much more involved test suite for bevy in the future, I propose a separation of concerns and would like to split these into two.

1. Leave ./tests/how_to_test_systems.rs as it currently is
2. Create a near-identical copy and keep in ./examples/testing which then could be elaborated on for user-friendliness if needed
3. Have this new one listed in ./examples/README.md and Cargo.toml

This way ./tests is free to change and evolve as needed without also having to be considered an approachable and user-friendly entry into testing within Bevy.

What alternative(s) have you considered?

• To add ./tests to the ignored folders of the example-validation action.
  This reduces cohesion and can create confusion for users as there exists examples that does not reside within ./examples

• Expand the example-doc-validation action to support multiple locations for examples to be in.
  This would further delay [Merged by Bors] - CI - Check that examples are listed in README and Cargo #1650, and again reduce cohesion.

[mockersf]

Assuming ./tests will become a much more involved test suite for bevy in the future,

There are no plans to increase much the ./tests folder

• We favour tests for each individual crate
• Having several test suites in there can severely impact build time and size as it creates a separate binary for each
• Integration tests for a game engine often means running with the renderer, but having cargo test actually starts a test game isn't nice

[Weibye]

Thanks for your input! I'm closing this as it is not the right solution, as discussed in #1650