Avoid calling @sock.close for CDP tests

[st0012]

@sock is only defined when running DAP tests:

debug/test/support/protocol_utils.rb

Lines 327 to 328 in 049db87

	def attach_to_dap_server
	@sock = Socket.unix @remote_info.sock_path

Therefore, @sock will be nil when finishing CDP tests and cause NoMethodError.

And we haven't seen errors because DAP tests are always ran before CDP's. So the @sock variable will be left assigned. If we use dap: false in any protocol test, we can see the error:

Error: test_detach_reattach_to_rdbg(DEBUGGER__::DetachTest):
  NoMethodError: undefined method `close' for nil:NilClass

        @sock.close
             ^^^^^^
/Users/st0012/projects/debug/test/support/protocol_utils.rb:271:in `ensure in execute_cdp_scenario'
/Users/st0012/projects/debug/test/support/protocol_utils.rb:274:in `execute_cdp_scenario'
/Users/st0012/projects/debug/test/support/protocol_utils.rb:47:in `run_protocol_scenario'
test/protocol/detach_test.rb:23:in `test_detach_reattach_to_rdbg'
     20:     RUBY
     21:
     22:     def test_detach_reattach_to_rdbg
  => 23:       run_protocol_scenario PROGRAM, dap: false do
     24:         assert_reattach
     25:         req_terminate_debuggee
     26:       end

[ono-max]

Thanks. I already fixed in #499.

[st0012]

@ono-max Ok, I'll close this. But do you think you can open a separate PR for the fix? Currently it's not possible to run only CDP tests and it bothers me when testing around the framework.