devpod: fix tests eval

Without the change `tests` eval failed as:

    $ nix build --no-link -f. devpod.tests
    error:
       error: value is a string while a set was expected

The failure reason is in `package` parameter. It should be a derivation,
not a package name. While at it fixed the command to extract version.

pkgs/development/tools/devpod/default.nix

@@ -63,7 +63,8 @@ rec {
     '';
 
     passthru.tests.version = testers.testVersion {
-      package = pname;
+      package = devpod;
+      command = "devpod version";
       version = "v${version}";
     };
   };