docs: include instructions on how to run without stack

For those who don't want Stack or cannot install it for any reason, we are interested in telling them what the alternative is. This alternative is the command `runghc -Wall -isrc test/Tests.hs`.

Offer this command in https://github.com/exercism/haskell/blob/main/docs/TESTS.md. We will need to modify https://github.com/exercism/haskell/blob/main/docs/INSTALLATION.md as well since that would mean Stack is no longer required.

Originally discussed in exercism/haskell#1043, but this issue description is intended to have provided complete context without having viewed the previous discussion.