TW-1584: attr.{isnt,not} use partial matching

- Change attr.{isnt,not} to be the exact opposite of attr{:is,:},
  i.e. exact matching.
- Fix tests that assumed the old behavior was intended.

src/CLI.cpp

@@ -1234,7 +1234,7 @@ void CLI::desugarFilterAttributeModifiers ()
                 }
                 else if (modifier == "isnt" || modifier == "not")
                 {
-                  op.attribute ("raw", "!=");
+                  op.attribute ("raw", "!==");
                   rhs.attribute ("raw", "'" + value + "'");
                   rhs.tag ("LITERAL");
                 }