Fix test harness where blocking could occur on IO read.

We had issues with the CI where some tests would sometimes hang
on cleanup when it would call VBoxManage, even with a timeout.
My only assumption is that this is happening as it waits on `readline`,
so we do a `read_nonblock` which will help immensely.

test/acceptance/support/isolated_environment.rb

@@ -98,12 +98,14 @@ module Acceptance
         if !readers.empty?
           begin
             readers.each do |r|
-              data = r.readline
+              data = r.read_nonblock(1024)
               io_data[r] += data
-              io_name = r == stdout ? "stdout" : "stderr"
-              @logger.debug("[#{io_name}] #{data.chomp}")
+              @logger.debug(data)
               yield io_name.to_sym, data if block_given?
             end
+          rescue IO::WaitReadable
+            # This just means the IO wasn't actually ready and we should
+            # wait some more. So we just let this pass through.
           rescue EOFError
             # Process exited, so break out of this while loop
             break