Add Spector test coverage for the CommonPropertiesErrorClient's methods

[antkmsft]

1. Do not modify the generated rust source code (in src/generated), only modify the emitter code (in /packages/typespec-rust/src/) if necessary
2. The test for the common-properties does already exist (under packages/typespec-rust/test/spector/azure/resource-manager/common-properties/tests/). Add a file called common_properties_error_client_test.rs there, and add tests for two methods - create_for_user_defined_error() and get_for_predefined_error().