Speed up the Solr test suite ~8.5x: 7:27 -> 0:53

[reekitconcept]

Fixes #81.

The Solr-based test suite became much slower after the unittest → pytest migration. Two verified causes, two fixes — tests/services goes from 7:27 to 0:53 (449 tests, green):

1. Keep the Plone test layers alive for the whole session (7:27 → 4:18)

zope.pytestlayer only preserves layers across test classes for zope.testrunner-style tests; for pytest-style tests its class-scoped layer fixture tore the whole layer stack down after every test class, rebuilding the Plone site many times per run (empirically: 8 layer setups in a two-directory run). An autouse session fixture depending on the generated *_session layer fixtures marks the layers keep-for-whole-session. Per-test isolation is unaffected.

2. Class-scoped content and query setup for the service tests (4:18 → 0:53)

With parametrized tests, a single Solr query followed by N assertions ran as N separate tests — each repeating the full setup (registry, index clear, content creation, commit/reindex) and the same query. The services fixture chain and the per-class _init fixtures move to class scope, on a new functional_class_bracket fixture that runs the layer's testSetUp/testTearDown once per test class (plone.testing layer resources stack LIFO, so per-function brackets still nest inside where used).

Verified with --setup-show: a parametrized class with 4 tests runs portal_with_content (content creation) and _init (the query) exactly once each — previously 4× each. Safety verified by AST scan: no test body mutates content or registry (all mutations live in fixtures), so sharing one ZODB/Solr state per class is sound.

Note: expect a tiny overlap with #85 on the solr_service fixture scope — resolves trivially whichever merges second.