[tests] Revisit unit tests for redundancy and consolidation (volume 2)

[derasdf]

Follow-up to #1506 (closed by PR #1526).

Volume 1 consolidated duplicates and removed IT-covered
wiring tests. There might be more to cull in the same
categories, plus stale test infrastructure and mock data
generation scripts worth looking into.

Current unit test counts (main, 1401 total)

• pip: 321
• yarn: 186
• gomod: 157
• models: 134
• npm: 105
• cli: 100
• yarn_classic: 103
• rpm: 47
• bundler: 24
• cargo: 22
• pnpm: 11
• other: 191