test: add override to ServerDone function

Currently the following compiler warning is displayed when building:

  ../test/cctest/test_inspector_socket_server.cc:142:8: warning:
  'ServerDone' overrides a member function but is not marked 'override'
        [-Winconsistent-missing-override]
    void ServerDone() {
         ^
  ../src/inspector_socket_server.h:30:16: note: overridden virtual
  function is here
    virtual void ServerDone() = 0;
                 ^

This commit marks ServerDone with override to get rid of the warning.

PR-URL: https://github.com/nodejs/node/pull/13166


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>