[clang-tidy][objc] Finds and fixes improper usages of XCTAssertEquals and XCTAssertNotEquals.

Using XCTAssertEqual on NSString* objects is almost always  wrong.

Unfortunately, we have seen a lot of tests doing this and reyling on pointer equality for strings with the same values (which happens to work sometimes - depending on the linker, but this assumption is not guaranteed by the language)

These fixes would make tests less brittle.

Differential Revision: https://reviews.llvm.org/D114975