During the work on [0], we asked TRex developers on the proper use, and they replied in [1].
Implement their suggestion.

This means not relying on scripts in resources/tools/trex/.
Either put the TRex client object into TrafficGenerator class, or create a separate class to hide TRex-specific insternals (such as xstats).

[0] https://gerrit.fd.io/r/c/csit/+/22120
[1] https://github.com/cisco-system-traffic-generator/trex-core/issues/341