test: verify issues #275, #279, #282 are not reproducible

[chaliy]

Summary

• Add regression tests confirming 3 reported issues work correctly
• bug: Parser corrupts \( in single-quoted strings inside while loops #275: \( in single-quoted strings preserved as literal
• bug: "done" cannot be used as a case pattern (treated as reserved word) #279: Reserved words like done work as case patterns
• bug: find -type f doesn't enumerate files in VFS directories #282: find -type f enumerates VFS files correctly

Test plan

• All 4 tests pass locally
• CI green

Closes #275 #279 #282