Document Test Scenarios in `regression` Directory

[lindluni]

In ./tools/PTE/CITests we have a README that lists our test tasks. We should replicate this in the root of the regression director and it should include a list of each of our tests, a very, very brief description of the test and a link to the README for each of our tests.

With that being said, each individual test directory should also include a README describing the test scenario and the topology of the test network.

This will help developers identify tests they want to target when they want to test something related to changes they've made.

All future tests will need to include an update to the global README and the README specific to the scenario