tests: fix further breakage due to upstream output changes

Mercurial changesets 10f15e34d86c and ca275f7ec576 added messages when
bookmarks are activated and deactivated, respectively.
import: remove redundant imports

These modules are already imported above.
verify: add a wrapper function in __init__.py

Upcoming patches will switch to the new-style command decorator instead of the
explicit command table. That doesn't mesh well with top-level command functions
defined in other modules.
commands: switch to new-style declaration via decorators

This is supported in all the versions that hg-git supports.
gimport: drop empty options and redundant synopsis, and add help text
gexport: drop empty options and redundant synopsis, and add help text
gclear: drop empty options and move help text to docstring

The synopsis is meant for a command description, not help text.
git-cleanup: drop empty options and move help text to docstring