add os_x? methods to TestUtils to fix broken tests in OS X

Because 2 tests failure.

Just fix failure tests in OS X environment.
```log
..............F
===============================================================================
Failure:
test_select_font_face(ContextTest)
/Users/cosmo/Github/rcairo/test/test_context.rb:78:in `test_select_font_face'
     75:     default_font_family = ""
     76:     # default_font_family = "Helvetica" if quartz?
     77:     default_font_family = "Arial" if win32?
  => 78:     assert_equal([default_font_family,
     79:                   Cairo::FONT_SLANT_NORMAL,
     80:                   Cairo::FONT_WEIGHT_NORMAL],
     81:                  [face.family, face.slant, face.weight])
<["", 0, 0]> expected but was
<["Helvetica", 0, 0]>

diff:
? ["Helvetica", 0, 0]
===============================================================================
.....F
===============================================================================
Failure:
test_toy_font_face_new(FontFaceTest)
/Users/cosmo/Github/rcairo/test/test_font_face.rb:13:in `test_toy_font_face_new'
     10:     default_font_family = ""
     11:     # default_font_family = "Helvetica" if quartz?
     12:     default_font_family = "Arial" if win32?
  => 13:     assert_equal([default_font_family,
     14:                   Cairo::FONT_SLANT_NORMAL,
     15:                   Cairo::FONT_WEIGHT_NORMAL],
     16:                  [face.family, face.slant, face.weight])
<["", 0, 0]> expected but was
<["Helvetica", 0, 0]>

diff:
? ["Helvetica", 0, 0]
===============================================================================
```