Partest redirects stderr to log file

Some scalac output is on stderr, and it's useful to see that
in the log file, especially for debugging.
Flags file with only -feature is not necessary.
Deprecations
Correct import
Check with useful true warning.
Feature required.
Feature not required.
Heed useful warning.
Ignore broken warnings.
Suppress deprecation by deprecating
Debug output.
Process destroy i/o threads