docs: include instructions on how to run without stack

[petertseng]

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 #1043, but this issue description is intended to have provided complete context without having viewed the previous discussion.