MDL-44891 Behat: Add step to check attribute

This commit creates a Behat step of the form:

Then the "title" attribute of "Toggle visibility" "button"
should contain "Show"

This is useful because unless I missed something there is currently
no step that checks attribute values; this seems like a simple
generic step. Behat is intended to check user-visible effects in
the resulting HTML, and sometimes these effects are implemented
using attribute values (two common examples are the alt= attribute,
which is visible to screenreader users, and the title= attribute,
which is visible to all users; there's also the src attribute of
images, and lots of other possibilities too).