fix: make test/ headers compliant with styleguide

These were not included in that check. Beats me as to why.

Change-Id: Ib44c7f0b10672359026f95daeb088039cf6df4eb

misc/pre-commit.nix

@@ -81,7 +81,7 @@ pre-commit-run {
     check-headers = {
       enable = true;
       package = pkgs.check-headers;
-      files = "^lix/";
+      files = "^(lix/|tests/)";
       types = [
         "c++"
         "file"

tests/unit/libstore/protocol.hh

@@ -1,3 +1,5 @@
+#pragma once
+/// @file
 #include <gtest/gtest.h>
 
 #include "lix/libutil/strings.hh"

tests/unit/libutil-support/tests/test-data.hh

@@ -1,4 +1,5 @@
 #pragma once
+/// @file
 
 #include "lix/libutil/types.hh"
 #include "lix/libutil/environment-variables.hh"