tests: add inject_randr15 (#2573)

tests: add inject_randr15 (#2573)

This tool is similar to xtrace in usage in that it intercepts traffic to
the X server. The motivating feature for writing the tool is its ability
to inject prepared reply messages instead of the server’s reply. In
this particular case, we’ll inject a RRGetMonitors reply to test i3’s
RandR 1.5 code paths.

The added testcase is a noop for now, but with the code that’s lingering
in the randr15 branch, i3 does actually detect monitors as per the
injected reply:

    2016-11-20 21:10:05 - randr.c:__randr_query_outputs:618 -
      RandR 1.5 available, querying monitors
    2016-11-20 21:10:05 - randr.c:__randr_query_outputs:628 -
      1 RandR monitors found (timestamp 0)
    2016-11-20 21:10:05 - randr.c:__randr_query_outputs:646 -
      name DP3, x 0, y 0, width 3840 px, height 2160 px, width 520 mm,
      height 290 mm, primary 1, automatic 1

This is preparation work for issue #1799