pathname: Improve absolute? tests along with relative? tests

Check that these two methods opposite always.

test/pathname/test_pathname.rb

@@ -260,13 +260,12 @@ class TestPathname < Test::Unit::TestCase
     assert_equal(Pathname("/foo/var"), r)
   end
 
-  def test_absolute
-    assert_equal(true, Pathname("/").absolute?)
-    assert_equal(false, Pathname("a").absolute?)
-  end
-
   def relative?(path)
-    Pathname.new(path).relative?
+    path = Pathname.new(path)
+    relative = path.relative?
+    absolute = path.absolute?
+    assert_equal(!relative, absolute)
+    relative
   end
 
   defassert(:relative?, true, '')
@@ -281,7 +280,7 @@ class TestPathname < Test::Unit::TestCase
   defassert(:relative?, !DOSISH_DRIVE_LETTER, 'A:/')
   defassert(:relative?, !DOSISH_DRIVE_LETTER, 'A:/a')
 
-  if File.dirname('//') == '//'
+  if DOSISH_UNC
     defassert(:relative?, false, '//')
     defassert(:relative?, false, '//a')
     defassert(:relative?, false, '//a/')