test($parse): ensure CSP code paths are used when testing

Previously, the test suite was not actually taking CSP-mode paths when we were expecting it to.

Numerous CSP-mode tests are failing, working on fixing these.
fix($parse): ensure CSP assignable expressions have `assign()`

Fixes regression where the `assign()` method was not added to chains of identifiers in CSP mode,
introduced originally in b3b476d.

Closes #9048