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/tmux/hm-session-vars.sh

@@ -1,5 +0,0 @@
-# Only source this once.
-if [ -n "$__HM_SESS_VARS_SOURCED" ]; then return; fi
-export __HM_SESS_VARS_SOURCED=1
-
-export TMUX_TMPDIR="${XDG_RUNTIME_DIR:-"/run/user/\$(id -u)"}"

tests/modules/programs/tmux/secure-socket-enabled.nix

@@ -11,9 +11,8 @@ with lib;
 
     nmt.script = ''
       assertFileExists home-path/etc/profile.d/hm-session-vars.sh
-      assertFileContent home-path/etc/profile.d/hm-session-vars.sh ${
-        ./hm-session-vars.sh
-      }
+      assertFileContains home-path/etc/profile.d/hm-session-vars.sh \
+        'export TMUX_TMPDIR="''${XDG_RUNTIME_DIR:-"/run/user/\$(id -u)"}"'
     '';
   };
 }