tests: move to t, allow yath test from root

By moving the tests subdirectory to t, we gain the ability to run `yath
test` with no arguments from inside `nix develop` in the root of the
the repo.

(`nix develop` is necessary in order to set the proper env vars for
`yath` to find our test libraries.)

t/jobs/succeed-with-failed.sh

@@ -0,0 +1,3 @@
+#! /bin/sh
+mkdir -p $out/nix-support
+touch $out/nix-support/failed