skip more non-compiling code snippets

doc/Language/math.pod6

@@ -279,7 +279,7 @@ parameter.
 How can we translate that into Perl 6? Well Math::Model bring some help
 with a very undestable way to do that, let's see it:
 
-=begin code
+=begin code :skip-test<can't use>
 use Math::Model;
 
 my $m = Math::Model.new(
@@ -363,7 +363,7 @@ captures    => ('x'),
 
 At this point our model is set. We need to run the simulation and render
  a cool plot about our results:
-=begin code
+=begin code :skip-test<Snippet for explanation purposes>
 $m.integrate(:from(0), :to(8), :min-resolution(0.5));
 $m.render-svg('population growth malthus.svg', :title('population growth'));
 =end code
@@ -394,7 +394,7 @@ where the constant g defines the growth rate and k is the carrying
  capacity.
 Modifying the above code we can simulate its behaviour in time:
 
-=begin code
+=begin code :skip-test<can't use>
 use Math::Model;
 
 my $m = Math::Model.new(
@@ -444,7 +444,7 @@ I<critical point>.
 
 Our code would be:
 
-=begin code
+=begin code :skip-test<can't use>
 use Math::Model;
 
 my $m = Math::Model.new(