run-tests: fix HGTESTEXTRAEXTENSIONS with py3
Since extensions was a str and section bytes, it never populated anything.
If it had, it would have put bytes into the environment dictionary that is all
str. As everything starts and ends as str, remove the incomplete attempt at
byteification. It doesn't appear that we had any test coverage of this bit of
code, so also add a non-extension config to make sure it is filtered out
Differential Revision: https://phab.mercurial-scm.org/D9557