Rollup merge of #143229 - jieyouxu:compiletest-maintenance-1, r=Kobzol

[COMPILETEST-UNTANGLE 1/N] Move some some early config checks to the lib and move the compiletest binary

This is part of a patch series to untangle `compiletest` to hopefully nudge it towards being more maintainable.

This PR:

- Moves some early config checks (some warnings) to the compiletest library.
- Moves `src/main.rs` to `src/bin/main.rs` to make the separation (as in, compiletest's library component vs the tool binary component) more obvious.

r? ``@Kobzol`` (or reroll)

Author: matthiaskrgr