[2.x] Adds the stress function for declaring stress tests

[lukeraymonddowning]

Much like the PR to the Arch plugin, this PR adds the stress function as a top level test creator that also marks the test as being part of the stress group, allowing the user (and Pest) to easily only run stress tests or exclude stress tests.

The problem with this plugin is that we already declare the stress function to start a stress test. I handle this in the following way:

• stress now accepts two parameters. If only one is passed, it acts as it did before. If both are passed, it acts as a test function.
• A new function has been added, visit, which performs the same functionality that stress did previously. The stress function now calls visit under the hood.

My hope is that we can encourage users to make use of visit instead of stress inside the test closure itself, although obviously what I've added here is still fully backwards compatible. Obviously I'm also open to other approaches here.

Kind Regards,
Luke