The problem with ignore_output is that it hides errors for WAY=ghci.
GHCi always returns with exit code 0 (unless it is broken itself).
For example: ghci015 must have been failing with compile errors for
years, but we didn't notice because all output was ignored.
Therefore, replace all uses of ignore_output with either ignore_stderr
or ignore_stdout. In some cases I opted for adding the expected output.
Update submodule hpc and stm.