Revert "Don't isolate tests as possible"

This reverts commit 40387a4f9c.

Reason: some tests fail on Rails 5.1

Rakefile

@@ -4,19 +4,13 @@ require "bundler/gem_tasks"
 
 task :default => :test
 
-# FIXME: Redefining :test task to run test/options_test.rb in isolated process since it depends on whether Rails is loaded or not.
+#FIXME: Redefining :test task to run each test in isolated process.
 # Remove this task when we finished changing escape_html option to be true by default.
-isolated_test = Rake::TestTask.new do |t|
-  t.libs << 'test'
-  t.test_files = %w[test/options_test.rb]
-  t.warning = true
-  t.verbose = true
-end
-Rake::TestTask.new do |t|
-  t.libs << 'test'
-  t.test_files = Dir['test/*_test.rb'] + Dir['test/haml-spec/*_test.rb'] - isolated_test.file_list
-  t.warning = true
-  t.verbose = true
+task :test do
+  test_files = Dir.glob('test/**/*_test.rb').reject { |f| f.start_with?('test/gemfiles/vendor/bundle') }
+  test_files.all? do |file|
+    sh(Gem.ruby, '-w', '-I/lib', '-Itest', file)
+  end || raise('Failures')
 end
 
 CLEAN.replace %w(pkg doc coverage .yardoc test/haml vendor)