[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.
* {
color: #000 !important;
}
div#popupLoginBox,
div#popupLoginBox,
img#logo, div.header,
#navColumnLeft, #navColumnRight,
#footer {
display: none
}
div#pageContent{
border: none;
}