MDL-44725 Update cm_info::create to allow 'false' param (13)

Currently cm_info::create allows the 'null' parameter, and
returns null (as a null $cm should still be null when treated
as a cm_info object).

Some unit tests relied on the value 'false' being treated
the same as null in this regard. This seems like a generally
safe assumption (given this is a function about changing
weakly typed data into a stronger type) so I modified the
function to accept anything PHP false, returning null.
MDL-44725 Add Behat support for setting grouping condition (14)

Makes it possible to add a grouping condition when creating an
activity, using the Behat field type API. You can set the field
using a syntax 'Grouping: NAME' (at present, only groupings are
supported, obviously we could potentially extend this to add
support for other conditions if needed).

Tested in Firefox and Chrome in JS mode, and in non-JS mode.