tests: check that all firefox derivatives can be installed

The issue itself was resolved with:
nix-community#6460

This patch only adds a test case to make sure we don't ever regress by
installing firefox and librewolf and floorp at the same time.

Closes: nix-community#6467

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

tests/modules/programs/firefox/floorp-with-firefox.nix

@@ -0,0 +1,16 @@
+{ config, lib, realPkgs, ... }:
+
+lib.mkIf config.test.enableBig {
+  programs.firefox = {
+    enable = true;
+    package = realPkgs.firefox-unwrapped;
+  };
+  programs.floorp = {
+    enable = true;
+    package = realPkgs.floorp-unwrapped;
+  };
+  programs.librewolf = {
+    enable = true;
+    package = realPkgs.librewolf-unwrapped;
+  };
+}

tests/modules/programs/firefox/floorp.nix

@@ -1 +1,3 @@
-import ./common.nix "floorp"
+import ./common.nix "floorp" // {
+  "floorp-with-firefox" = ./floorp-with-firefox.nix;
+}