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/deepgit-builder.sh

@@ -0,0 +1,6 @@
+#! /bin/sh
+
+set -e
+mkdir $out
+cp -v $src/* $out/
+git describe --long > $out/Version