tests: Should we follow stylistic conventions more consistently?

In test code, we've historically been less strict about enforcing global stylistic conventions. We generally expect consistency within a test, but different tests vary in:

• using namespace std; versus std:: qualification (there are a relatively small number of tests where this makes a real difference)
• const T versus T const
• class versus typename in template parameters

Historically, this seemed reasonable because it allowed each maintainer to write test code in their most preferred style, and we generally don't treat the test code as a unified whole (we generally write a test, and then it stays unchanged unless we need to fix/extend it). However, this makes things somewhat confusing for contributors, since we don't have consistent precedents to follow.

No files, tests, or entry points are identified in the issue. Review the issue discussion first to determine whether a consistent convention is wanted and which stylistic choices are in scope; done requires an explicit decision and an identified set of test changes.