fix(acl): keep the integration tests out of Stryker's solution (refs #174)

Stryker runs every test project in Acl.slnx regardless of test-projects, so
Acl.IntegrationTests (needs a live OpenZaak) failed 8 tests in every initial
run. Acl.slnx is read only by Stryker; the root solution still builds it.
Re-measured with a green initial run: 90.45 %, so break: 90 stands.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

services/acl/Acl.slnx

@@ -1,7 +1,9 @@
 <Solution>
+  <!-- Stryker-only. Acl.IntegrationTests is left out on purpose: it needs a live
+       OpenZaak, so in the mutation job it fails its initial run (#174). The root
+       register-referentie.slnx still builds and lints it. -->
   <Project Path="Acl.Api/Acl.Api.csproj" />
   <Project Path="Acl.Application/Acl.Application.csproj" />
   <Project Path="Acl.Infrastructure/Acl.Infrastructure.csproj" />
-  <Project Path="Acl.IntegrationTests/Acl.IntegrationTests.csproj" />
   <Project Path="Acl.Tests/Acl.Tests.csproj" />
 </Solution>