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/basic.nix

@@ -0,0 +1,20 @@
+with import ./config.nix;
+{
+  empty_dir =
+    mkDerivation {
+      name = "empty-dir";
+      builder = ./empty-dir-builder.sh;
+    };
+
+  fails =
+    mkDerivation {
+      name = "fails";
+      builder = ./fail.sh;
+    };
+
+  succeed_with_failed =
+    mkDerivation {
+      name = "succeed-with-failed";
+      builder = ./succeed-with-failed.sh;
+    };
+}