Load extension ('json/ext') consistently in test_ext

[aeroastro]

Summary

This patch fixes the issue that rake test does not actually test
C extension for CRuby.

The Bug

The test task is defined as follows:

json/Rakefile

Lines 109 to 110 in 7ec73ad

	desc "Testing library (pure ruby and extension)"
	task :test => [ :test_pure, :test_ext ]

This invokes :test_pure and :test_ext consecutively.

:test_pure assigns ENV['JSON'] = 'pure' as follows:

json/Rakefile

Lines 97 to 98 in 7ec73ad

	task :test_pure => [ :set_env_pure, :check_env, :do_test_pure ]
	task(:set_env_pure) { ENV['JSON'] = 'pure' }

However, :test_ext for CRuby does not change ENV['JSON'].

json/Rakefile

Line 252 in 7ec73ad

task :test_ext => [ :check_env, :compile, :do_test_ext ]

This lead to require 'json/pure' in test_helper.rb, which test only the pure version.

json/tests/test_helper.rb

Lines 2 to 4 in 7ec73ad

	when 'pure'
	$:.unshift File.join(__dir__, '../lib')
	require 'json/pure'

How to Fix It

I have added :set_env_ext to guarantee that :test_ext assigns ENV['JSON'] = 'ext'.
This is just like test_ext in JRuby version