TST: merge high_memory mark with single_cpu
- [ ] [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
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

3/4 tests that were using `@pytest.mark.high_memory` were already using `@pytest.mark.single_cpu`, so may as well combine them.

hmm can you rebase