Add fail mode to test_hook.rb

spec/test_hook.rb

@@ -4,6 +4,7 @@
 output = []
 $stdin.readlines.each do |line|
   exit 0 if line.chomp == '#clear'
+  exit 1 if line.chomp == '#fail'
   output << line.gsub(/([^\s]*)\s*=\s*(.*)/, '\1 = \2')
 end
 

spec/test_hook_spec.rb

@@ -14,6 +14,10 @@ describe 'Test Hook' do
     expect(out).to eq ''
   end
 
+  it 'fails with a #fail line' do
+    expect { run '#fail' }.to raise_error
+  end
+
   def run(input)
     result = IO.popen ['ruby', "#{__dir__}/test_hook.rb"], mode: File::RDWR do |io|
       io.write input