Bug 977588 - [Dialer] The Active call screen does not have the option to place calls on hold.

Include a test checking that the merge button is disabled when adding second call. You can take https://github.com/gtorodelvalle/gaia/commit/b258a29c3dedcb73ec2d3ae0efc9d29bca90e020#diff-4691c1b55512f896e6453a62dc49ca10R409 as a reference ;)