twister: Fix test-only tests with hardware map fixtures

In test-only mode with a hardware map, all tests with fixtures were
skipped, even when the hardware map has appropriate fixtures defined.

TestPlan.load_from_file had the check for fixtures missing, but actually
best is that the TestInstance checks the fixtures in the hardware map.

Signed-off-by: Manoel Brunnen <manoel.brunnen@ithinx.io>