Run compiler unit tests without stage 1 build

[cramertj]

./x.py test src/libsyntax --stage 0 builds all the way through a stage 1 build before actually running the tests, so any changes to libsyntax require rebuilding all of rustc in order to rerun the unit tests. This is a really slow process, making it much harder to write and run unit tests for rustc code.

[jyn514]

I think this is a duplicate of #44293.