Fix spec runner: read command output during run

IO.pipe buffers 4096 bytes. If the command output fills that up,
Process.waitpid will hang waiting for the spawned process to finish.

Read command output from the pipe in a thread. This thread will self-exit
when the pipe is closed.

Fixes #1287

spec/support/tests/command_runner.rb

@@ -25,6 +25,7 @@ module Tests
 
     def initialize(*args)
       @reader, @writer = IO.pipe
+      @command_output = ''
       options = (args.last.is_a?(Hash) ? args.pop : {})
       @args = args
       @options = options.merge(
@@ -84,7 +85,7 @@ module Tests
     def output
       @_output ||= begin
         stop
-        without_colors(reader.read)
+        without_colors(command_output)
       end
     end
 
@@ -126,7 +127,7 @@ Output:
 
     protected
 
-    attr_reader :args, :reader, :writer, :wrapper
+    attr_reader :args, :command_output, :reader, :writer, :wrapper
 
     private
 
@@ -149,8 +150,23 @@ Output:
 
     def run
       pid = spawn(env, *command, options)
+      t = Thread.new do
+        loop do
+          begin
+            @command_output += reader.read_nonblock(4096)
+          rescue IO::WaitReadable
+            IO.select([reader])
+            retry
+          rescue EOFError
+            break
+          end
+        end
+      end
       Process.waitpid(pid)
       @status = $?
+    ensure
+      writer.close unless writer.closed?
+      t.join
     end
 
     def run_with_wrapper