sdk: Add REQUIRE_COUCHDB_INTEGRATION_TESTS to fail CouchDB tests

[paul-gerber-svg]

CouchDB tests in test/backend/test_couchdb.py are skipped whenever
no CouchDB is reachable. In CI, the sdk-test job always starts a
CouchDB service container, so a broken setup there just skipped the
tests instead of failing the build.

This adds a REQUIRE_COUCHDB_INTEGRATION_TESTS environment variable,
same idea as REQUIRE_SERVER_INTEGRATION_TESTS for the server's
Docker integration tests: set it to fail instead of skip when no
CouchDB is reachable. CI now sets it for the sdk-test job.

Also fixes the setup_testdb.py docstring, which said CI passes
--failsafe to avoid failing on an unreachable CouchDB, which was false.

[zrgt]

• Since #639 is merged, this now conflicts in test/_helper/test_helpers.py and in the unittest step of pr.yml. Please rebase onto develop and keep both flags (REQUIRE_SCHEMA_TESTS and REQUIRE_COUCHDB_INTEGRATION_TESTS). Ideally both should share one small helper to parse the env var instead of duplicating .lower() in {"1", "true", "yes"}.
• test/examples/test_tutorials.py also uses skipUnless(COUCHDB_OKAY) for the CouchDB tutorial test, so it can still skip silently in CI. Could you cover it with the same flag?