tests: Do not load vimwiki/taskwiki twice

Since c8c3cc9a00, vimwiki filetype is set
automatically by :edit, and `set filetype=vimwiki` loads everything
again.

One consequence of loading taskwiki twice is that
taskwiki/testcoverage.py is sourced twice, the Coverage object is
initialized, started, stopped and saved twice (with the same data
filename!) and the coverage report is inaccurate as two trackers write
into the same file.

tests/base.py

@@ -88,7 +88,6 @@ class IntegrationTest(object):
         self.add_plugin('vimwiki', 'plugin/vimwiki.vim')
         self.filepath = os.path.join(self.dir, 'testwiki.txt')
         self.client.edit(self.filepath)
-        self.command('set filetype=vimwiki')
 
     def teardown(self):
         if not self.client: