Fix tests

* Fix deprecation warnings from tests
* Refactor tests so that ´@testset` is usually called in master file
  `runtests.jl`, not inside test file. Later on we can convert tests
  to examples.
* Syntax of tests now follow more closely syntax used currently in
  JuliaFEM. We have had earlier studies with different kind of syntaxes,
  now we have kind of explicit way to do things.

test/test_virtual_work.jl

@@ -1,8 +1,7 @@
 # This file is a part of JuliaFEM.
 # License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
 
-using JuliaFEM
-using JuliaFEM.Testing
+using JuliaFEM, Test
 
 abstract type PlaneStressElasticityProblem <: AbstractProblem end
 
@@ -61,9 +60,8 @@ function test_residual_form()
     free_dofs = [3, 4, 5, 6]
     solve!(problem, free_dofs, 0.0)  # launch a newton solver for single element
     disp = element("displacement", [1.0, 1.0], 0.0)
-    info("displacement at tip: $disp")
+    @info("displacement at tip: $disp")
 
     # verified using Code Aster.
     @test isapprox(disp[2], -8.77303119819776E+00)
 end
-