keys: Add test script.

keys: Add test script.

I've recently found a number of bugs in this area, so it's probably
worth writing a test script to make sure they stay dead.

This wouldn't be complete without some edits to the tripe-keys script,
so (a) make it executable in the build tree, and (b) flush output before
running subprocesses so that the output appears in the correct order
even when it's redirected to a file.