discourse.tests: Test the appropriate discourse package

Perform the tests on the package that the `tests` attribute is a child
of, i.e. if `discourseAllPlugins.tests` is built, the tests will run
with the `discourseAllPlugins` package, not the `discourse` package as
previously.

(cherry picked from commit 6fd5a40ccaf0b4da1362803a387bf46d381dd66a)

nixos/tests/discourse.nix

@@ -4,7 +4,7 @@
 #  3. replying to that message via email.
 
 import ./make-test-python.nix (
-  { pkgs, lib, ... }:
+  { pkgs, lib, package ? pkgs.discourse, ... }:
   let
     certs = import ./common/acme/server/snakeoil-certs.nix;
     clientDomain = "client.fake.domain";
@@ -55,7 +55,7 @@ import ./make-test-python.nix (
 
         services.discourse = {
           enable = true;
-          inherit admin;
+          inherit admin package;
           hostname = discourseDomain;
           sslCertificate = "${certs.${discourseDomain}.cert}";
           sslCertificateKey = "${certs.${discourseDomain}.key}";