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
...