Re-enable example_base_md_reftraj_restart.py

[yakutovicha]

When run by GitHub actions, the test example_base_md_reftraj_restart.py keeps failing. I couldn't reproduce this behaviour locally, so I decided to temporarily disable it in #222 until we figured out the cause.

An example of the failed test can be found here (unless removed by GitHub).

For the record, the output with the error looks like so:

examples/workchains/example_base_md_reftraj_restart.py:185: SystemExit
----------------------------- Captured stdout call -----------------------------
Testing CP2K MD REFTRAJ on H2 (DFT) through a workchain...
Submitted calculation...
OK, EXT_RESTART section is present in the final_input_parameters.
ERROR, stepids are NOT correct. Expected: [ 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20] but got:  [19 20  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18]