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-products.nix

@@ -0,0 +1,14 @@
+with import ./config.nix;
+{
+   simple =
+     mkDerivation {
+      name = "build-product-simple";
+      builder = ./build-product-simple.sh;
+    };
+   
+   with_spaces = 
+     mkDerivation {
+      name = "build-product-with-spaces";
+      builder = ./build-product-with-spaces.sh;
+    };
+}