ci: gate on the fast tests (about 20s); slow end-to-end tests run in parallel with the image build [CI]

backend/tests/MoShiro.Backend.Tests/HostTests.cs

@@ -221,6 +221,7 @@ public sealed class HostStatusTests
     }
 
     [Fact]
+    [Trait("Category", "Slow")]
     public async Task AProbeThatTurnsReportingOff_SendsNothing()
     {
         await _backend.SiteMustBePublicAsync();