CombineTest and CombineFnsTest should not rely on behavior of opaque TestCombineFn

This is a CombineFn that "exhaustively" uses all the capabilities of Combine. It results in readability problems for tests. Each test should clearly set things up and check particular functionality.

Imported from Jira [BEAM-2050](https://issues.apache.org/jira/browse/BEAM-2050). Original Jira may contain additional context.
Reported by: kenn.

Start by reading CombineTest, CombineFnsTest, and the TestCombineFn behavior they currently exercise. Identify each test's particular Combine capability, then make its setup and assertions explicit rather than relying on the opaque helper. Done means the tests clearly verify individual functionality without depending on exhaustive helper behavior.

Written by the indexing model from the issue text.