MDEV-26949 --debug-gdb installs redundant signal handlers

There is a server startup option --gdb a.k.a. --debug-gdb that requests
signals to be set for more convenient debugging. Most notably, SIGINT
(ctrl-c) will not be ignored, and you will be able to interrupt the
execution of the server while GDB is attached to it.

When we are debugging, the signal handlers that would normally display
a terse stack trace are useless.

When we are debugging with rr, the signal handlers may interfere with
a SIGKILL that could be sent to the process by the environment, and ruin
the rr replay trace, due to a Linux kernel bug
https://lkml.org/lkml/2021/10/31/311

To be able to diagnose bugs in kill+restart tests, we may really need
both a trace before the SIGKILL and a trace of the failure after a
subsequent server startup. So, we had better avoid hitting the problem
by simply not installing those signal handlers.

mysql-test/lib/My/Debugger.pm

@@ -147,7 +147,8 @@ sub do_args($$$$$) {
   my %vars = (
     vardir => $::opt_vardir,
     exe => $$exe,
-    args => join(' ', map { quote_from_mtr $_ } @$$args, '--loose-gdb'),
+    args => join(' ', map { quote_from_mtr $_ } @$$args,
+                 '--loose-debug-gdb', '--loose-skip-stack-trace'),
     input => $input,
     script => "$::opt_vardir/tmp/${k}init.$type",
     log => "$::opt_vardir/log/$type.$k",

sql/mysqld.cc

@@ -9740,7 +9740,6 @@ static int get_options(int *argc_ptr, char ***argv_ptr)
   {
     /* Allow break with SIGINT, no core or stack trace */
     test_flags|= TEST_SIGINT;
-    opt_stack_trace= 1;
     test_flags&= ~TEST_CORE_ON_SIGNAL;
   }
   /* Set global MyISAM variables from delay_key_write_options */