Update tests for Asciidoctor 1.5.0

The default Docbook backend in Asciidoctor 1.5.0 has been changed to
docbook5.  The existing Docbook test in `tilt_asciidoctor_test.rb`
expects Docbook 4.5 output, so it should use the `docbook45` backend.  I
added a new test with different expected output for the `docbook5`
backend.

There is no longer a test for the `docbook` backend since its output can
change depending on the installed version of Asciidoctor.