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/runcommand.nix

@@ -0,0 +1,33 @@
+with import ./config.nix;
+{
+  metrics = (
+    mkDerivation {
+      name = "my-build-product";
+      builder = "/bin/sh";
+      outputs = [ "out" "bin" ];
+      args = [
+        (
+          builtins.toFile "builder.sh" ''
+            #! /bin/sh
+
+            echo "$PATH"
+
+            mkdir $bin
+            echo "foo" > $bin/bar
+
+            metrics=$out/nix-support/hydra-metrics
+            mkdir -p "$(dirname "$metrics")"
+            echo "lineCoverage 18 %" >> "$metrics"
+            echo "maxResident 27 KiB" >> "$metrics"
+          ''
+        )
+      ];
+    }
+  ) // {
+    meta = {
+      license = "GPL";
+      description = "An example meta property.";
+      homepage = "https://github.com/NixOS/hydra";
+    };
+  };
+}