More systematic compile/run markup for "pass" UI tests

[petrochenkov]

TLDR:

In test files:

// check-pass
// build-pass
// run-pass

In `x.py` command line (forcing):

--force =check
--force >=check
--force <=check
--force =build
--force >=build
--force <=build
--force =run
--force >=run
--force <=run

In `x.py` command line (running a subset):

--select =check
--select >=check
--select <=check
--select =build
--select >=build
--select <=build
--select =run
--select >=run
--select <=run

TODO:
More detailed description, motivation, etc.

cc @Centril

[Centril]

I've added:

    /// Only run tests that match any of these modes.
    pub filter_mode: Option<Vec<Mode>>,

to Config.

The thinking is that you pass --filter-mode run-pass,compile-fail,... and have only those tests run.

A --filter-mode scheme seems easiest to implement given the current compiletest infrastructure and doesn't require us to rename a bunch of annotations.

[petrochenkov]

After some thought, perhaps filtering is not even necessary with --force check.
I'll make some experiments and report (sometime this week).