[ArrayManager] TST: activate a bunch of (passing) tests
xref https://github.com/pandas-dev/pandas/issues/39146/

This is on top of #40152, so only the second commit is relevant: https://github.com/pandas-dev/pandas/pull/40217/commits/e7209aaa7b85c206d3efcd446eaa54b34851cb98

But this basically adds the following to CI for ArrayManager:

```
        pytest pandas/tests/api/ --array-manager
        pytest pandas/tests/base/ --array-manager
        pytest pandas/tests/computation/ --array-manager
        pytest pandas/tests/config/ --array-manager
        pytest pandas/tests/dtypes/ --array-manager
        pytest pandas/tests/generic/ --array-manager
        pytest pandas/tests/indexes/ --array-manager
        pytest pandas/tests/libs/ --array-manager
        pytest pandas/tests/plotting/ --array-manager
        pytest pandas/tests/scalar/ --array-manager
        pytest pandas/tests/strings/ --array-manager
        pytest pandas/tests/tools/ --array-manager
        pytest pandas/tests/tseries/ --array-manager
        pytest pandas/tests/tslibs/ --array-manager
        pytest pandas/tests/util/ --array-manager
```