fix($parse): add `.assign()` function to settable expressions in CSP mode

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

Numerous CSP-mode tests are failing, working on fixing these.
In angular core, `$sniffer.csp` is a function --- so we want to call the function to find out if we're in CSP mode or not. But if people ever decorate it, they might decorate it with primitives, so it tries both.