Improve ftrace_config_muxer_unittest

Misc improvements to ftrace_config_muxer_unittest:
* Use a mock instead of global state for AtraceWrapper. This is why this
  refactoring started: a future CL might add new mehtods to
  AtraceWrapper, so I thought it would be easier to use dependency
  injection properly.
* Move the FtraceConfigMuxer dependencies into fixtures as much as
  possible.
* Construct FtraceConfigMuxer into fixtures, where possible. This makes
  it easier to change the constructor.
* As a result there are three separate fixtures now: one that just
  provides some dependencies, one that builds a FtraceConfigMuxer with a
  fake proto translation table and one that builds a FtraceConfigMuxer
  with a mock proto translation table.

Change-Id: Ia2808d95a203b9af77a507e90e53377aff68e901