Add `--enable-tests` for cabal-install setup instructions

The command line given to get the sandbox package database path is missing quotation which may make it fail in some shells (given certain globbing options). This adds single quotation marks to avoid such potential issues.

[ci-skip]

For some reason github is finicky when it comes to local links. Also,
I'm dyslexic it seems when it comes to Markdown's link syntax.

* Reformat tests/README, add links to individual files.
* Add link to tests/README to HACKING.
* Add links to license files.

* Ensure README.md is in .cabal files.
* Remove some verbiage.
* Remove broken link to old wiki in HACKING.

* Attempt to fix local links.
* Experiment with syntax highlighting.
* Add title to HACKING.
* Reword an awkward sentence and a few wordy headings.
* Change "Unix" to "Unix-like".

* Rename README files to README.md and link from root.
* Rename HACKING to HACKING.md and link from root.
* Add additional links.
* A fair amount of added punctuation and rewording.
* Replace references to darcs/trac with git/github.

Seems people do not know that the source guide exists.