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/git-rev-input.nix

@@ -0,0 +1,10 @@
+with import ./config.nix;
+{ src }:
+{
+  copy =
+    mkDerivation {
+      name = "git-rev-input";
+      builder = ./scm-builder.sh;
+      inherit src;
+    };
+}