[gdb] Add maint selftest -verbose option

[gdb] Add maint selftest -verbose option

The print_one_insn selftest in gdb/disasm-selftests.c contains:
...
  /* If you want to see the disassembled instruction printed to gdb_stdout,
     set verbose to true.  */
  static const bool verbose = false;
...

Make this parameter available in the maint selftest command using a new option
-verbose, such that we can do:
...
(gdb) maint selftest -verbose print_one_insn
...

Tested on x86_64-linux.