`ASSERT_TRUE` the parsed `ModuleOp` outside the `ParseModuleOpString` call.
`ASSERT_TRUE` the parsed `ModuleOp` outside the `ParseModuleOpString` call.

Current implementation contains an `EXPECT_TRUE` inside `ParseModuleOpString` to check whether the created `OwningOpRef<ModuleOp>` is valid. However, using `ASSERT_TRUE` is more suitable because downstream tests would crash otherwise, if the returned module op is not valid.
Since `ASSERT_*` cannot be placed inside a function, this change takes out the check outside of the `ParseModuleOpString` call.
