In k5test, look for lldb if gdb is not found

XCode for macOS provides lldb but not gdb.  For convenience, make
k5test default to lldb if gdb is not found in the path.

src/util/k5test.py

@@ -575,8 +575,7 @@ def _parse_args():
     parser.add_option('--debug', dest='debug', metavar='NUM',
                       help='Debug numbered command (or "all")')
     parser.add_option('--debugger', dest='debugger', metavar='COMMAND',
-                      help='Debugger command (default is gdb --args)',
-                      default='gdb --args')
+                      help='Debugger command (default is gdb --args)')
     parser.add_option('--stop-before', dest='stopb', metavar='NUM',
                       help='Stop before numbered command (or "all")')
     parser.add_option('--stop-after', dest='stopa', metavar='NUM',
@@ -589,12 +588,21 @@ def _parse_args():
     verbose = options.verbose
     testpass = options.testpass
     _debug = _parse_cmdnum('--debug', options.debug)
-    _debugger_command = shlex.split(options.debugger)
     _stop_before = _parse_cmdnum('--stop-before', options.stopb)
     _stop_after = _parse_cmdnum('--stop-after', options.stopa)
     _shell_before = _parse_cmdnum('--shell-before', options.shellb)
     _shell_after = _parse_cmdnum('--shell-after', options.shella)
 
+    if options.debugger is not None:
+        _debugger_command = shlex.split(options.debugger)
+    elif which('gdb') is not None:
+        _debugger_command = ['gdb', '--args']
+    elif which('lldb') is not None:
+        _debugger_command = ['lldb', '--']
+    elif options.debug is not None:
+        print('Cannot find a debugger; use --debugger=COMMAND')
+        sys.exit(1)
+
 
 # Translate a command number spec.  -1 means all, None means none.
 def _parse_cmdnum(optname, str):