Avoid double sending of SIGINT to subprocesses (#889)

Standard bash behavior when pressing Ctrl+C is to send SIGINT to the
foreground process group. This combines well with standard fork behavior
because subprocesses are started in the same group, so a multi-process
program is correctly terminated without any explicit signal handling.

It however does not work well in cases where signals are handled
explicitly like here, because on Ctrl+C the subprocesses receive two
SIGINTs - one directly from the shell and one from the interrupt handler
in the `CLI` class. This in turn can prevent graceful shutdown in the
subprocesses.

The specific example that prompted this fix was interrupting feature
specs using selenium. In case of a double interrupt the browser process
will sometimes remain running after rspec is terminated.

lib/parallel_tests/test/runner.rb

@@ -110,7 +110,7 @@ def print_command(command, env)
         def execute_command_and_capture_output(env, cmd, options)
           pid = nil
 
-          popen_options = {}
+          popen_options = { pgroup: true }
           popen_options[:err] = [:child, :out] if options[:combine_stderr]
 
           output = IO.popen(env, cmd, popen_options) do |io|