[RegTest] Show stderr in verbose mode

[curiousdannii]

If error messages are printed to stderr, then even in verbose mode nothing is shown.

[erkyrath]

We're not capturing stderr in the subprocess.Popen() calls. Doesn't it go straight out to the script's stderr?

[curiousdannii]

No, it just seems to be disappearing.

[curiousdannii]

Another thing that would help with errors in Remglk mode would be to detect when a stdin line doesn't begin with {, and output it instead of just failing to parse JSON and timing out.

[erkyrath]

I went back to test this with a hacked RemGlk+Glulxe containing the line

fprintf(stderr, "### this is a stderr line\n");

It comes through on a regtest run, with or without -v:

% python3 regtest.py advent.regtest 
* base
### this is a stderr line
* save
### this is a stderr line
* menu
### this is a stderr line

% python3 regtest.py advent.regtest -v > /dev/null
### this is a stderr line
### this is a stderr line
### this is a stderr line

[erkyrath]

Another thing that would help with errors in Remglk mode would be to detect when a stdin line doesn't begin with {, and output it instead of just failing to parse JSON and timing out.

I've added this:

* base
NotJSONException, interpreter output:
  ### this is a stdout line

[curiousdannii]

It comes through on a regtest run, with or without -v:

That's good. It really seemed like it wasn't working before. If I ever run into the problem again I'll try to get more details about exactly what I'm doing.