Added assertions for realpath and realdirpath

It is said that realpath(3) and realdirpath(3) on some platforms
may return a relative path.

test/ruby/test_file.rb

@@ -252,6 +252,10 @@ class TestFile < Test::Unit::TestCase
       tst = realdir + (File::SEPARATOR*3 + ".")
       assert_equal(realdir, File.realpath(tst))
       assert_equal(realdir, File.realpath(".", tst))
+      assert_equal(realdir, Dir.chdir(realdir) {File.realpath(".")})
+      realpath = File.join(realdir, "test")
+      File.write(realpath, "")
+      assert_equal(realpath, Dir.chdir(realdir) {File.realpath("test")})
       if File::ALT_SEPARATOR
         bug2961 = '[ruby-core:28653]'
         assert_equal(realdir, File.realpath(realdir.tr(File::SEPARATOR, File::ALT_SEPARATOR)), bug2961)
@@ -318,6 +322,8 @@ class TestFile < Test::Unit::TestCase
       assert_equal(realdir, File.realdirpath(tst))
       assert_equal(realdir, File.realdirpath(".", tst))
       assert_equal(File.join(realdir, "foo"), File.realdirpath("foo", tst))
+      assert_equal(realdir, Dir.chdir(realdir) {File.realdirpath(".")})
+      assert_equal(File.join(realdir, "foo"), Dir.chdir(realdir) {File.realdirpath("foo")})
     }
     begin
       result = File.realdirpath("bar", "//:/foo")