[devtools] make migration handler easily available from CubicWebTC through .shell() method
So we are able to use shell/migration helper methods directly from test cases.
<div id="ajaxroot">
<div class="ajaxHtmlHead">
<script src="http://foo.js" type="text/javascript"> </script>
<link rel="stylesheet" type="text/css" media="all" href="qunit.css" />
</div>
<h1>Hello</h1>
</div>