[docs] Better documentation on how to run tests

Very little is documented on how to run tests locally, and what caveats exist to doing so unless a contributor is already familiar with the process.

in light of the proposed change in https://github.com/stacks-network/stacks-core/pull/6927 , which adds a new bitcoin testing env var, i think it makes sense to document how to run tests locally (notably, i don't see it documented in markdown when the required BITCOIND_TEST=1 is needed).

ref: https://github.com/stacks-network/stacks-core/blob/master/CONTRIBUTING.md?plain=1#L350

cc @federico-stacks @dhaney-stacks