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/api-test.nix

@@ -0,0 +1,13 @@
+let
+  thisFile = builtins.toFile "default.nix" (builtins.readFile ./default.nix);
+  builder = builtins.toFile "builder.sh" ''
+    echo ${thisFile} > $out
+  '';
+in {
+  job = derivation {
+    name = "job";
+    system = builtins.currentSystem;
+    builder = "/bin/sh";
+    args = [ builder ];
+  };
+}