tests: add automated tests for VFS/mount bug fixes

tests: add automated tests for VFS/mount bug fixes

Added 4 new automated tests in fulltest.c:
- I16: MS_REMOUNT - verify remount updates flags correctly
- I17: Mount replace rejection - verify mount without MS_REMOUNT fails
- I18: ftruncate readonly - verify ftruncate respects MS_RDONLY
- I19: Mountpoint validation - verify mount fails on non-existent or non-directory mountpoints

Updated test harnesses:
- smoke_test.exp: added 4 new test patterns (119→123 tests)
- test_battery.exp: added 4 new test patterns (27→31 tests)

Added missing errno constants (EBUSY, ENOENT, ENOTDIR) to fulltest.c

Test results:
- Smoke test: 123/123 PASS
- Zero regressions