servstate: unify log assert and done check

The startTestServices() test helper uses a special entry in the
service command under test to write the service standard output
also to a log file that can be inspected.

This mechanism suffers from a race condition (as highlighted in
#264) because when the
content of the log file is loaded, the service may not yet have
completed writing to the log.

Since standard output is also verified separately through a
different mechanism, the following changes are made:

- Enhance the global "done check" (previous called "done file") to
  check completion per service. This effectively adds the
  capability previously provided by the log assert mechanism.

- Use the existing "done check" mechanism to wait until the service
  command-line is complete up to the point of the check.

- Only now verify the stdout buffer content as checked previously.

- Remove the log mechanism all together.