keymap-util: add tests and fix one small bug

keymap-util: add tests and fix one small bug

When converting an empty x11 variant, we would not delete vconsole mapping
properly.

find_legacy_keymap() is made non-static. I think it's important to be able to
test it. In principle we could also test it through the higher-level interface
of x11_convert_to_vconsole, but x11_convert_to_vconsole also uses
find_converted_keymap, and it's better to test at this lower level.

Note that find_legacy_keymap might be a bit of a misnomer, because we'd probably
want to keep kbd-model-map even if the "legacy" layouts went away.  So we might
want to change this name, but I'm leaving that for another commit.