Use omit instead of skip

This test runs with test/unit now, which defines omit instead of
skip.

test/io/console/test_io_console.rb

@@ -35,7 +35,7 @@ def test_failed_path
       Errno.const_get(e) if Errno.const_defined?(e)
     }
     exceptions.compact!
-    skip if exceptions.empty?
+    omit if exceptions.empty?
     File.open(IO::NULL) do |f|
       e = assert_raise(*exceptions) do
         f.echo?
@@ -226,7 +226,7 @@ def test_setecho2
   end
 
   def test_getpass
-    skip unless IO.method_defined?("getpass")
+    omit unless IO.method_defined?("getpass")
     run_pty("p IO.console.getpass('> ')") do |r, w|
       assert_equal("> ", r.readpartial(10))
       sleep 0.1
@@ -412,7 +412,7 @@ def test_sync
   def helper
     m, s = PTY.open
   rescue RuntimeError
-    skip $!
+    omit $!
   else
     yield m, s
   ensure
@@ -423,7 +423,7 @@ def helper
   def run_pty(src, n = 1)
     r, w, pid = PTY.spawn(EnvUtil.rubybin, "-I#{TestIO_Console::PATHS.join(File::PATH_SEPARATOR)}", "-rio/console", "-e", src)
   rescue RuntimeError
-    skip $!
+    omit $!
   else
     if block_given?
       yield r, w, pid