Move checking of tests with little x-platform risk to supplemental auto bot, turn on lexer tests

[brson]

We have tests in tree that the lexer conforms to an official grammar, but the bots are not running them yet. These tests take a long time to run, as will the impending tests of the parser. We need to do a few things to turn them on:

• Port grammar and lexer tests to rustbuild from grammar.mk
• Add a make check-extra in src/bootstrap/mk/makefile.in that runs the grammar and lexer tests. Submit PR to rust-lang/rust
• Create a Dockerfile that is capable of running the tests
• Make modifications to rust-buildbot that a new auto-extra builder
• Submit PR to rust-buildbot.

cc @cmr @alexcrichton

[emberian]

The lexer tests aren't too bad, but the parser tests are going to be awful.

[emberian]

It looks like the Ubuntu antlr packages are not-very-useful, will have to install from their website. It's missing the grun tool entirely, which is the only thing that lets me avoid actually writing Java for the lexer tests.

[edunham]

Help wanted: Create Docker image with all dependencies to run these tests, and take a guess at the Buildbot PR necessary to run the new image as a non-gating builder.

[alexcrichton]

Grammar bot is #28592 and otherwise we basically have a supplemental bot running extra tests.