Add tests for COG_WHEEL env var scenarios
Test coverage for all COG_WHEEL values:
- Default with cog_runtime: false -> embedded cog wheel
- Default with cog_runtime: true -> coglet-alpha (PinnedCogletURL)
- COG_WHEEL=cog -> embedded cog wheel (overrides cog_runtime)
- COG_WHEEL=coglet -> embedded coglet wheel
- COG_WHEEL=coglet-alpha -> PinnedCogletURL
- COG_WHEEL=https://... -> URL install (with/without coglet env vars)
- COG_WHEEL=/path/to/file.whl -> local file install
Closes cog-6q8
