Tests: Add forgotten HTools.ExtLoader test

Found by cabal.

Test and test data were apparently forgotten.

There was an unused import as well.

Signed-off-by: Niklas Hambuechen <niklash@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>
Signed-off-by: Niklas Hambuechen <niklash@google.com>