(forked from  issue 779267 )

VMTest is failing consistently on betty-release. As a speculative fix[1], I made VMTest and TastVMTest run serially, which had the effect of making TastVMTest not run at all when VMTest fails.

I should rework SimpleBuilder's _RunVMTests method to run TastVMTestStage even if VMTestStage throws an exception. It sounds like failures_lib.CompoundFailure is the way to do this.

1. which didn't work, or at least didn't fix all the failures