Add modification tests for immediate value

mrbgems/mruby-metaprog/test/metaprog.rb

@@ -44,6 +44,8 @@ assert('Kernel#instance_variable_set', '15.3.1.3.22') do
   %w[@6 @% @@a @ a].each do |n|
     assert_raise(NameError) { o.instance_variable_set(n, 1) }
   end
+  assert_raise(FrozenError) { o.freeze.instance_variable_set(:@a, 2) }
+  assert_raise(FrozenError, ArgumentError) { nil.instance_variable_set(:@a, 2) }
 end
 
 assert('Kernel#instance_variables', '15.3.1.3.23') do
@@ -121,6 +123,8 @@ assert('Kernel#define_singleton_method') do
   end
   assert_equal :test_method, ret
   assert_equal :singleton_method_ok, o.test_method
+  assert_raise(TypeError) { 2.define_singleton_method(:f){} }
+  assert_raise(FrozenError) { [].freeze.define_singleton_method(:f){} }
 end
 
 assert('Kernel#singleton_class') do

test/t/kernel.rb

@@ -419,6 +419,7 @@ assert('Kernel#remove_instance_variable', '15.3.1.3.41') do
   assert_raise(NameError) { tri.remove }
   assert_raise(NameError) { tri.remove_instance_variable(:var) }
   assert_raise(FrozenError) { tri.freeze.remove }
+  assert_raise(FrozenError, NameError) { :a.remove_instance_variable(:@v) }
 end
 
 # Kernel#require is defined in mruby-require. '15.3.1.3.42'