gh-109276: libregrtest calls random.seed() before each test (#109279)

libregrtest now calls random.seed() before running each test file
when -r/--randomize command line option is used. Moreover, it's also
called in worker processes. It should help to make tests more
deterministic. Previously, it was only called once in the main
process before running all test files and it was not called in worker
processes.

* Convert some f-strings to regular strings in test_regrtest when
  f-string is not needed.
* Remove unused all_methods variable from test_regrtest.
* Add RunTests members are now mandatory.