tests: improve test purity

Before the XDG variables would be set from the user's environment, if
available. This would break some tests.

With this change the tests should be fully deterministic.

Fixes #1190

tests/modules/programs/bash/session-variables-expected.txt

@@ -1,6 +1,6 @@
 # -*- mode: sh -*-
 
-. "/home/testuser/.nix-profile/etc/profile.d/hm-session-vars.sh"
+. "/home/hm-user/.nix-profile/etc/profile.d/hm-session-vars.sh"
 
 export V1="v1"
 export V2="v2-v1"

tests/modules/programs/bash/session-variables.nix

@@ -13,8 +13,6 @@ with lib;
       };
     };
 
-    home.homeDirectory = "/home/testuser";
-
     nmt.script = ''
       assertFileExists home-files/.profile
       assertFileContent \