Fix type of various sessionVariables options

Unfortunately, using `attrsOf` is not possible since it results in too
eager evaluation. In particular, the

    home.sessionVariables = {
      FOO = "Hello";
      BAR = "${config.home.sessionVariables.FOO} World!";
    };

example will cause an infinite recursion.

This commit restores the option type of

- `home.sessionVariables`,
- `pam.sessionVariables`,
- `programs.bash.sessionVariables`, and
- `programs.zsh.sessionVariables`

to `attrs`. It also adds test cases for the above options to avoid
regressions.

Fixes #659

modules/misc/pam.nix

@@ -14,7 +14,7 @@ in
   options = {
     pam.sessionVariables = mkOption {
       default = {};
-      type = with types; attrsOf (either int str);
+      type = types.attrs;
       example = { EDITOR = "vim"; };
       description = ''
         Environment variables that will be set for the PAM session.