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/config.nix.in

@@ -0,0 +1,9 @@
+rec {
+  path = "@testPath@";
+
+  mkDerivation = args:
+    derivation ({
+      system = builtins.currentSystem;
+      PATH = path;
+    } // args);
+}