m/pkg/supervisor: propagate panic in TestHarness

Before this change, the following test passes, which is not what you 
would expect:

func TestHarnessPanic(t *testing.T) {
	TestHarness(t, func(ctx context.Context) error {
		panic("PANIC!")
	})
}

Change-Id: I3356241c022a75a552f8338e532196fb9100a307
Reviewed-on: https://review.monogon.dev/c/monogon/+/2920
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>
Vouch-Run-CI: Serge Bazanski <serge@monogon.tech>
diff --git a/metropolis/pkg/supervisor/supervisor_testhelpers.go b/metropolis/pkg/supervisor/supervisor_testhelpers.go
index 1a72ad8..d3f994f 100644
--- a/metropolis/pkg/supervisor/supervisor_testhelpers.go
+++ b/metropolis/pkg/supervisor/supervisor_testhelpers.go
@@ -67,7 +67,7 @@
 			return err
 		}
 		return nil
-	}, WithExistingLogtree(lt))
+	}, WithExistingLogtree(lt), WithPropagatePanic)
 
 	t.Cleanup(func() {
 		ctxC()