texlive: remove upstream dependency in tests

This changes the tests to not require downloading the texlive
distribution.

tests/modules/programs/texlive/texlive-minimal.nix

@@ -1,4 +1,4 @@
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 with lib;
 
@@ -6,8 +6,22 @@ with lib;
   config = {
     programs.texlive.enable = true;
 
+    # Set up a minimal mocked texlive package set.
+    nixpkgs.overlays = [
+      (self: super: {
+        texlive = {
+          collection-basic = pkgs.writeTextDir "collection-basic" "";
+          combine = tpkgs:
+            pkgs.symlinkJoin {
+              name = "dummy-texlive-combine";
+              paths = attrValues tpkgs;
+            };
+        };
+      })
+    ];
+
     nmt.script = ''
-      assertFileExists home-path/bin/tex
+      assertFileExists home-path/collection-basic
     '';
   };
 }