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

tests/run-tests.py

@@ -1364,14 +1364,14 b' class Test(unittest.TestCase):'
 
         extraextensions = []
         for opt in self._extraconfigopts:
-            section, key = _sys2bytes(opt).split(b'.', 1)
+            section, key = opt.split('.', 1)
             if section != 'extensions':
                 continue
-            name = key.split(b'=', 1)[0]
+            name = key.split('=', 1)[0]
             extraextensions.append(name)
 
         if extraextensions:
-            env['HGTESTEXTRAEXTENSIONS'] = b' '.join(extraextensions)
+            env['HGTESTEXTRAEXTENSIONS'] = ' '.join(extraextensions)
 
         # LOCALIP could be ::1 or 127.0.0.1. Useful for tests that require raw
         # IP addresses.

tests/test-run-tests.t

@@ -1956,9 +1956,12 b' Test automatic pattern replacement'
   $ cat << EOF >> test-config-opt.t
   >   $ hg init test-config-opt
   >   $ hg -R test-config-opt purge
+  >   $ echo "HGTESTEXTRAEXTENSIONS: \$HGTESTEXTRAEXTENSIONS"
+  >   HGTESTEXTRAEXTENSIONS: purge
   > EOF
 
-  $ rt --extra-config-opt extensions.purge= test-config-opt.t
+  $ rt --extra-config-opt extensions.purge= \
+  >    --extra-config-opt not.an.extension=True test-config-opt.t
   running 1 tests using 1 parallel processes 
   .
   # Ran 1 tests, 0 skipped, 0 failed.