[mlir][emitc] Clean up EmitC tests (#152327)
Changes:
- Unnecessary `-verify-diagnostics` flags were removed from tests.
- `mlir/test/mlir-translate/emitc_classops.mlir` was moved to
`mlir/test/Target/Cpp/class.mlir`.
- The `transfrom.mlir` test was renamed to `form-expressions.mlir` for
clarity.
- Test for `emitc.class`, `emitc.field` and `emitc.get_field` was added
to `mlir/test/Dialect/EmitC/ops.mlir`.
- `wrap_emitc_func_in_class.mlir` and
`wrap_emitc_func_in_class_noAttr.mlir` were combined into
`wrap-func-in-class.mlir`.
