test(codersdk/agentsdk): fix context cancel flush test (#10560)

This change tests that the patch request is cancelled instead of hoping
that there's no race between context cancellations leading to patch
never being called.

codersdk/agentsdk/logs_test.go

@@ -338,9 +338,12 @@ func TestStartupLogsSender(t *testing.T) {
 		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitShort)
 		defer cancel()
 
-		var want, got []agentsdk.Log
+		patchStart := make(chan struct{})
-		patchLogs := func(_ context.Context, req agentsdk.PatchLogs) error {
+		patchDone := make(chan struct{})
-			got = append(got, req.Logs...)
+		patchLogs := func(ctx context.Context, _ agentsdk.PatchLogs) error {
+			close(patchStart)
+			<-ctx.Done()
+			close(patchDone)
 			return nil
 		}
 
@@ -358,10 +361,14 @@ func TestStartupLogsSender(t *testing.T) {
 		})
 		require.NoError(t, err)
 
-		cancel()
+		go func() {
+			<-patchStart
+			cancel()
+		}()
 		err = flushAndClose(ctx)
 		require.Error(t, err)
 
-		require.Equal(t, want, got)
+		<-patchDone
+		// The patch request should have been canceled if it was active.
 	})
 }