Make ZIP extract test clearer, add test case where only unzip can be used

[Leonidas-from-XIV]

This moves things around in the test a bit:

• Fake extractor binaries are in a separate folder, so they don't need to be written multiple times if used in multiple tests
• show_path shows the environment before execution to make it easier to see what's in PATH
• Use (PATH=.fakebin …) consistently
• Remove unzip --version, it's never called
• Add a test for when there's just unzip and dune can't fall back to bsdtar.

[Sudha247]

LGTM! Thanks for cleaning up the test.