test: compile test module only on test

[bamorim]

Sorry for not even opening an issue, I realized that just opening a PR and explaining would be easier and better for the contribution.

So, when looking at the source code I realize this repository was compiling a module intended only to test it's typespec generation. The problem with that is that then this is shipped to all codebases that use this library.

Another problem is that this adds those modules to the generated ex_docs, which can make people wonder whether or not they are supposed to understand their purpose.

This PR solves this problem by moving that file to a test/support directory and by setting the elixirc_paths to include test/support only on test environments (test and mysql).

[bamorim]

Any updates on that? @gjaldon