mtr --gdb: fix for --rr and for a warning

use _RR_TRACE_DIR=dir instead of -o dir, as the former can store
multiple traces in dir (if, e.g., the test restarts mysqld)

suppress uninitialized warning when $exe is undefined (--manual-XXX)

mysql-test/lib/My/Debugger.pm

@@ -74,7 +74,8 @@ my %debuggers = (
     options => '-f -o {log} {exe} {args}',
   },
   rr => {
-    options => 'record -o {log} {exe} {args}',
+    options => '_RR_TRACE_DIR={log} rr record {exe} {args}',
+    run => 'env',
     pre => sub {
       ::mtr_error('rr requires kernel.perf_event_paranoid <= 1')
         if ::mtr_grab_file('/proc/sys/kernel/perf_event_paranoid') > 1;