tests: --update-data incompatible with xdist

[ikonst]

Fixes a gotcha with --update-data.

When using --update-data with parallelized tests (xdist), the line offsets determined at test time may become wrong by update time, as multiple workers perform their updates to the same file.

This makes it so we exit with a usage error when --update-data is used with parallelism.

[ikonst]

For completeness: Another option would be to rewrite --update-data so that:

1. when it enqueues changes, enqueue "update section Y of testcase X to Z" rather than "update line range A..B to Z".
2. when it applies changes, parse the INI structure (right now we simply load the file as list[str]) and rewrite while replacing specific section(s)
   2.1. while holding a lock over the file

This is obviously more complex.

[A5rocks]

Thank you for catching my gotcha! I (as someone who doesn't know our own pytest configuration) assumed numprocesses wouldn't be defined, but now I see that this definitely works.