Simplify test_value_counts_unique_nunique_null in pandas/tests/base/test_ops.py

As stated by @jreback in [his comment](https://github.com/pandas-dev/pandas/pull/32046#discussion_r383021295) on #32046, there are some weird constructs, duplications and unreachable code in this test.

For more info see the mentioned PR and the unresolved comments in the bottom.

The other tests in this file have a similar problem, but should probably be a separate PR.