test: minor updates to improve virtiofs pass rate

[benhillis]

Resolve a few minor issues preventing some virtiofs tests from passing.

1. An incorrect error check when trying to map an app execution alias. This was causing the test to segfault and exit early. Was able to track this down because of https://github.com/microsoft/WSL/pull/13811.
2. Some test logic that needed to be updated to account for what virtiofs supports.