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 properly. Differential Revision: https://phab.mercurial-scm.org/D9557