tests: add a comment that we're purposely testing py2 extension attributes

Avoid someone unknowingly removing test coverage.  There are tests for a
properly byteified `testedwith` a few lines down.  I don't see similar for
`buglink`, but it's a trivial conversion to bytes, so I'm not concerned about
testing the expected/wanted extension state.

Differential Revision: https://phab.mercurial-scm.org/D9434

tests/test-extension.t

@@ -1423,7 +1423,8 b' No declared supported version, extension'
   ** Mercurial Distributed SCM * (glob)
   ** Extensions loaded: throw
 
-empty declaration of supported version, extension complains:
+empty declaration of supported version, extension complains (but doesn't choke if
+the value is improperly a str instead of bytes):
   $ echo "testedwith = ''" >> throw.py
   $ hg --config extensions.throw=throw.py throw 2>&1 | egrep '^\*\*'
   ** Unknown exception encountered with possibly-broken third-party extension throw
@@ -1434,7 +1435,8 b' empty declaration of supported version, '
   ** Mercurial Distributed SCM (*) (glob)
   ** Extensions loaded: throw
 
-If the extension specifies a buglink, show that:
+If the extension specifies a buglink, show that (but don't choke if the value is
+improperly a str instead of bytes):
   $ echo 'buglink = "http://example.com/bts"' >> throw.py
   $ rm -f throw.pyc throw.pyo
   $ rm -Rf __pycache__