test: update test-bundle2-format.t comment

Now that bundle2 is used by default for all exchanges, this comment is obviously
out of date. Having deep testing of the API and expected behavior of the format
and its processing is still valuable, so the comment is updated.

tests/test-bundle2-format.t

@@ -9,8 +9,8 b' Create an extension to test bundle2 API'
   $ cat > bundle2.py << EOF
   > """A small extension to test bundle2 implementation
   > 
-  > Current bundle2 implementation is far too limited to be used in any core
-  > code. We still need to be able to test it while it grow up.
+  > This extension allows detailed testing of the various bundle2 API and
+  > behaviors.
   > """
   > 
   > import sys, os, gc