xsession: add option importedVariables

This option lists the environment variables to be imported into the
systemd user session.

Also add a basic test of the xsession module.

tests/modules/misc/xsession/basic.nix

@@ -0,0 +1,30 @@
+{ config, lib, ... }:
+
+with lib;
+
+{
+  config = {
+    home.homeDirectory = "/test-home";
+
+    xsession = {
+      enable = true;
+      windowManager.command = "window manager command";
+      importedVariables = [ "EXTRA_IMPORTED_VARIABLE" ];
+      initExtra = "init extra commands";
+      profileExtra = "profile extra commands";
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.xprofile
+      assertFileContent \
+        home-files/.xprofile \
+        ${./basic-xprofile-expected.txt}
+
+      assertFileExists home-files/.xsession
+      assertFileContent \
+        home-files/.xsession \
+        ${./basic-xsession-expected.txt}
+
+    '';
+  };
+}