support: Add optstring support

This patch adds an option to test to add small command line option
through CMDLINE_OPTSTRING define.  For instance:

  #define CMDLINE_OPTSTRING "vd"

  static void
  cmdline_process_function (int c)
  {
    switch (c):
      'v':
        /* process '-v' option.  */
      break;
      'd':
        /* process '-d' option.  */
      break;
  }
  #define CMDLINE_PROCESS cmdline_process_function

It will add both '-v' and '-d' along with already default long options.

	* support/support_test_main.c (support_test_main):  Use optstring
	member for option string in getopt_long.
	* support/test-driver.c: Add comment about CMDLINE_OPTSTRING.
	(CMDLINE_OPTSTRING): New define.
	* support/test-driver.h (test_config): Add optstring member.