Extract coverage-specific code out of compiletest::runtest
          #125719
        
          
      
                
     Merged
            
            
          
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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.)