add basic mesh refinement test

test/runtests.jl

@@ -28,3 +28,24 @@ Random.seed!(0)
 
     @test img == img_sampled
 end
+
+@testset "mesh refinement" begin
+    # simple hashing
+    val(f) = integrate(f.space.mesh, (x; f) -> dot(f, f); f)
+
+    mesh = init_grid(zeros(5, 5))
+
+    f = FeFunction(FeSpace(mesh, P1(), (2, 3)))
+    f.data .= rand(size(f.data)...)
+
+    for i = 1:2
+        # refine roughly half of all cells
+        cells_ref = Set(findall(_ -> rand(Bool), cells(mesh)))
+
+        mesh_new, (f_new,) = refine(mesh, cells_ref; f)
+
+        @test isapprox(val(f), val(f_new))
+
+        (mesh, f) = (mesh_new, f_new)
+    end
+end