[Ironruby-core] Case sensitivity

[Sanghyeon Seo]

There are still some places in IronRuby that depends on case
insensitive file system.

1. :compile_testhost task generates IronRubyTestHost.exe, but
:test_compiler task runs ironrubytesthost.exe.
2. test_*.rb requires util/simple_test.rb, but SVN has Util directory instead.
3. mspec.rb requires builtin/string, etc, but SVN has Builtin/String
directory instead.

test_dir.rb tries to chdir to C:/... I can comment it out for the time being.

-- 
Seo Sanghyeon