 TST/CoW: copy-on-write tests for add_prefix and add_suffix
[Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature

    All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

This PR is related to https://github.com/pandas-dev/pandas/issues/49473
`add_suffix` and `add_prefix` already had Copy-on-Write implemented, this PR will add test cases that explicitly tests the Copy-on-Write feature.
thx @andrewchen1216 