update test to count number of points per extrema

test/runtests.jl

@@ -271,9 +271,15 @@ end
         @test fs |> extrema |> collect |> diff |> first > 1.9
     end
 
-    let f = sinc, int = (0, 40)  # JuliaPlots/Plots.jl/issues/3894
-        xs, fs = adapted_grid(f, int)
-        @test length(xs) > 400
+    let f = sinc, int = (-40, 40)  # JuliaPlots/Plots.jl/issues/3894
+        xs, fs = adapted_grid(sinc, int)
+        roots = vcat(int[1]:-1, 1:int[2])
+        count_per_extrema = map(1:length(roots)-1) do idx
+            left = roots[idx]; right = roots[idx+1]
+            return count(x -> left < x < right, xs)
+        end
+        # check that we have at least 5 points for each extrema
+        @test all(count_per_extrema .>= 5)
     end
 end