[XLA:GPU] Remove structured matchers for `SymbolicTile` tests and use string matcher instead.
[XLA:GPU] Remove structured matchers for `SymbolicTile` tests and use string matcher instead.

Also filter out the printing of the string "rt_vars" when the symbolic tile
does not contain any runtime variable.

This is in preparation for adding support for constraints, which would have
required yet another matcher without this change.
