While we might never be able to have 100% of our code strict, we can at
the very least do it for all of our tests. This ensures that our tests
are using our API with the types as intended by the test author, and not
silently be cast to what our API requires.
* Using htmlspecialchars to fix issue #3145: this prevents the SimpleXMLElement error "unterminated entity reference" when HTML entities are present in a worksheet
* replace htmlspecialchars with proposal of oleibman
* eliminate string cast, because PhpStan has a problem with that
* ignore string cast for phpstan + enhance unit test