Rename incremental cfail/cpass revisions to bfail/bpass

Long ago, UI tests were divided into "compile" and "run" tests. Later, the
compile tests were further subdivided into "check" and "build" tests, to speed
up tests that don't need a full build.

The same split was never applied to incremental test revisions, so the only way
to perform a check build in incremental tests is (confusingly) to use a `cfail`
revision and then specify `//@ check-fail` or `//@ check-pass`.

This PR makes room for dedicated check-fail and check-pass revisions by
renaming the existing `cfail` and `cpass` revisions to `bfail` and `bpass`,
since they currently perform a full build.

Author: Zalathar

src/tests/compiletest.md

@@ -158,8 +158,8 @@ then runs the compiler for each revision, reusing the incremental results from p
 
 The revisions should start with:
 
-* `cfail` — the test should fail to compile
-* `cpass` — the test should compile successully
+* `bfail` — the test should fail to compile
+* `bpass` — the test should compile successully
 * `rpass` — the test should compile and run successfully
 
 To make the revisions unique, you should add a suffix like `rpass1` and `rpass2`.