Fix regression reported by Michael Esser and Andrew Watson

[delcypher]

(independently).

In our recently switch to llvm::raw_ostream (and friends) (I think this
is d934d98) we forgot to flush the
llvm::raw_string_ostream to the underlying string used for error report
files (e.g. test000001.overshift.err) so we would end up writing an
empty string to error report files.

Also added a test case to catch this.

[ccadar]

Looks good to me. I have a more compact solution, but I'm not sure it's worth bothering:

- interpreterHandler->processTestCase(state, MsgString.c_str(), suffix);
+interpreterHandler->processTestCase(state, msg.str().c_str(), suffix);

(msg.str() does a flush first)

[delcypher]

@ccadar That is simplier. I've added an extra commit to do this

Assuming you're happy with this please commit this. Or if you'd like me to commit please give me commit access again (I removed my commit access some time ago).

[ccadar]

Looks ok, but you forgot to remove the flush(). Can you also collapse the three commits to Executor.cpp into a single one? Thanks.

[delcypher]

Done