new build_helper::compiletest module for safer invocations of compiletest by bootstrap

[lolbinarycat]

currently bootstrap just uses strings to represent various compiletest arguments, such as mode.

obviously it would be better to use enums for these, as they are much less vulnerable to typos.

compiletest already has enum definitions for these (in common.rs), and the two already have a shared dependency (build_helper), and build_helper already has access to serde.

i would be willing to work on this refactor, if people think it would be a good idea.

[onur-ozkan]

It would definitely be good to have a stricter type mechanism that is shared between bootstrap and compiletest.