lib/warnings.t: Adapt to warnings-by-default

[jkeenan]

In this ticket we will track our progress on implementing Objective 3, warnings-by-default, for lib/warnings.t in the core distribution. See that ticket for guidance on addressing warnings.

This file spews so many warnings that it will be useful to treat it separately from the overall ticket for the `lib' directory, #306.

Current output for this file can be seen here.

Acceptance Criteria: All tests for this subdirectory PASS; no tests emit warnings.

[jkeenan]

I'll be working on this file in the gha-322-lib-warnings-t-20201218 branch.