Rollup merge of #125719 - Zalathar:run-coverage, r=jieyouxu

Extract coverage-specific code out of `compiletest::runtest`

I had been vaguely intending to do this for a while, but seeing #89475 on the compiletest dashboard inspired me to actually go and do it.

This moves a few hundred lines of coverage-specific code out of the main module, making navigation a bit easier. There is still a small amount of coverage-specific logic in broader functions in that module, since it can't easily be moved.

This is just cut-and-paste plus fixing visibility and imports, so no functional changes.

I also removed the unit test for anonymizing line numbers in MC/DC reports, as foreshadowed by the comment I wrote when adding it. That functionality is now adequately exercised by the actual snapshot tests for MC/DC coverage.

(Removing the test now avoids the need to move it, or to make the function it calls visible.)

Author: matthiaskrgr