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/build-output-as-input.nix

@@ -0,0 +1,18 @@
+with import ./config.nix;
+let
+  jobs = {
+    build1 =
+      mkDerivation {
+        name = "build1";
+        builder = ./empty-dir-builder.sh;
+      };
+
+    build2 =
+      { build1 }:
+      mkDerivation {
+        name = "build2";
+        builder = ./empty-dir-builder.sh;
+        inherit build1;
+      };
+  };
+in jobs