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

tests/modules/programs/bash/default.nix

@@ -0,0 +1,3 @@
+{
+  bash-session-variables = ./session-variables.nix;
+}

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

@@ -0,0 +1,8 @@
+# -*- mode: sh -*-
+
+. "@homeDirectory@/.nix-profile/etc/profile.d/hm-session-vars.sh"
+
+export V1="v1"
+export V2="v2-v1"
+
+

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

@@ -0,0 +1,28 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.bash = {
+      enable = true;
+
+      sessionVariables = {
+        V1 = "v1";
+        V2 = "v2-${config.programs.bash.sessionVariables.V1}";
+      };
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.profile
+      assertFileContent \
+        home-files/.profile \
+        ${
+          pkgs.substituteAll {
+            src = ./session-variables-expected.txt;
+            inherit (config.home) homeDirectory;
+          }
+        }
+    '';
+  };
+}