Fix a failing test (#13)

* Fix a failing test

This commit fixes the following failure:

```
  1) Failure:
Fiddle::TestImport#test_no_message_with_debug [/Users/mrkn/src/github.com/ruby/fiddle/test/fiddle/test_import.rb:152]:

1. [2/2] Assertion for "stderr"
   | <[]> expected but was
   | <["Exception `NameError' at /Users/mrkn/.rbenv/versions/2.5.1/lib/ruby/2.5.0/fiddle/import.rb:157 - uninitialized constant Fiddle::Function::STDCALL"]>.
```

* Stop using Bundler.with_clean_env

* Clear existing Ruby environment variables on test_no_message_with_debug

Author: mrkn

test/fiddle/test_import.rb

@@ -149,7 +149,18 @@ def test_atof
     end
 
     def test_no_message_with_debug
+      # disable all Ruby environment variables
+      orig_RUBYOPT, ENV['RUBYOPT'] = ENV['RUBYOPT'], nil
+      orig_RUBYLIB, ENV['RUBYLIB'] = ENV['RUBYLIB'], nil
+
+      # load development fiddle instead of bundled one
+      libdir = File.expand_path('../../../lib', __FILE__)
+      ENV['RUBYLIB'] = libdir if File.file?(File.join(libdir, "fiddle/import.rb"))
+
       assert_in_out_err(%w[--debug --disable=gems -rfiddle/import], 'p Fiddle::Importer', ['Fiddle::Importer'])
+    ensure
+      ENV['RUBYLIB'] = orig_RUBYLIB
+      ENV['RUBYOPT'] = orig_RUBYOPT
     end
   end
 end if defined?(Fiddle)