Enhance TestmonXdistSync registration and initialization

[jogendra-india]

Description

Fixes #259

This PR fixes the sqlite3.OperationalError: database is locked error that occurs when using --testmon --testmon-nocollect with pytest-xdist.

Problem

When --testmon-nocollect is used, TestmonXdistSync was not registered because it was inside the if should_collect: block. This meant:

1. Workers didn't receive testmon_exec_id from the controller via pytest_configure_node hook
2. Each worker called TestmonData.for_local_run() (write mode) instead of for_worker() (read-only)
3. Multiple workers raced for the SQLite write lock → database locked error

Solution

1. Move TestmonXdistSync registration to the if should_select or should_collect: block, so it's registered whenever testmon is active
2. Add should_collect parameter to TestmonXdistSync.__init__() to track whether collection is active
3. Skip sync_db_fs_tests() in pytest_xdist_node_collection_finished() when in nocollect mode (since DB is read-only)

Changes

testmon/pytest_testmon.py

• Modified register_plugins() to register TestmonXdistSync in the if should_select or should_collect: block
• Added should_collect parameter to TestmonXdistSync.__init__()
• Updated pytest_xdist_node_collection_finished() to check self._should_collect before syncing

tests/test_xdist_integration.py (new file)

• Added tests for xdist integration with testmon
• Added regression test for the database lock issue

Testing

# Run the new tests
pytest tests/test_xdist_integration.py -v

# Run with tox (xdist environment)
tox -e xdist-py310

Backward Compatibility

This change is fully backward compatible:

• should_collect parameter defaults to True
• Existing behavior is preserved when --testmon-nocollect is not used
• The fix only affects the xdist + nocollect combination

Checklist

• Code changes fix the issue
• Added test case to prevent regression
• Existing tests pass
• Backward compatible

[tarpas]

I don't see the test file in the commit. (Force add it pls)

I think I would delete most of the code comments.