Add test to verify includes work

t/Config/include.t

@@ -0,0 +1,26 @@
+use strict;
+use Setup;
+
+my %ctx = test_init(
+    use_external_destination_store => 0,
+    hydra_config                   => "include foo.conf"
+);
+
+write_file($ctx{'tmpdir'} . "/foo.conf", q|
+<foo>
+  include bar.conf
+</foo>
+|);
+
+write_file($ctx{'tmpdir'} . "/bar.conf", q|
+  bar = baz
+|);
+
+require Hydra::Helper::Nix;
+use Test2::V0;
+
+is(Hydra::Helper::Nix::getHydraConfig(), {
+    foo => { bar => "baz" }
+}, "Nested includes work.");
+
+done_testing;