fix: test: Fix pytest-xdist loadscope splitting on "::" in params

LoadScopeScheduling._split_scope() uses rsplit("::", 1) to
extract the test file scope from a node ID.  When parametrized
test values contain "::" (IPv6 addresses like "cafe:cafe::cafe"
or "::1"), the split lands inside the parameter instead of at
the .py:: boundary.  This creates spurious scopes that get
assigned to different workers, each triggering a full fixture
setup (starting named instances).

Override _split_scope() in conftest.py to split on ".py::"
which is unambiguous.

Six tests in synthrecord/tests_synthrecord.py are affected.
A verification script is included in util/.

Assisted-by: Claude:claude-opus-4-7
Merge branch 'mnowak/fix-xdist-loadscope-split' into 'main'

See merge request isc-projects/bind9!12103