Refine message to `skip` in nested `assert`

- I think "Info" is used only to `skip`, so change to "Skip".
- Changed the default value of `assert` and specify the argument explicitly
  at the caller of `assert` because it is unnatural "Assertion failed" is
  output even though the assertion doesn't fail.

== Example:

  def assert_foo(exp, act)
    assert do
      assert_equal exp[0], act[0]
      assert_equal exp[1], act[1]
    end
  end

  def assert_bar(exp, act)
    assert do
      skip
    end
  end

  def assert_baz(exp, act)
    assert do
      assert_equal exp, act
      assert_bar exp, act
    end
  end

  assert 'test#skip_in_nested_assert' do
    assert_baz 1, 1
  end

  === Before this patch:

    ?..
    Info: test#skip_in_nested_assert (core)
     - Assertion[1]
        Info: Assertion failed (core)
         - Assertion[1-2]
            Skip: Assertion failed (core)
      Total: 3
         OK: 2
         KO: 0
      Crash: 0
    Warning: 0
       Skip: 1

  === After this patch:

    ???
    Skip: test#skip_in_nested_assert (core)
     - Assertion[1]
        Skip: assert (core)
         - Assertion[1-2]
            Skip: assert (core)
      Total: 3
         OK: 0
         KO: 0
      Crash: 0
    Warning: 0
       Skip: 3

1 files changed, 1 insertions, 1 deletions

diff --git a/mrbgems/mruby-complex/test/complex.rb b/mrbgems/mruby-complex/test/complex.rb
index 5b7c3bfa4..4ae80515b 100644
--- a/mrbgems/mruby-complex/test/complex.rb
+++ b/mrbgems/mruby-complex/test/complex.rb
@@ -1,5 +1,5 @@
 def assert_complex(real, exp)
-  assert do
+  assert "assert_complex" do
     assert_float real.real,      exp.real
     assert_float real.imaginary, exp.imaginary
   end