[lldb][test] Break early when walking backtrace in concurrent tests

We only need to see that 1 frame of the stack is in user code. No need
to carry on looking.

Doing so actually caused a test failure on Armv8 Ubuntu Jammy where
a libc function does not have a display name. I'm sure I'm going to
get stung by this elsewhere, but for this test, breaking early
sidesteps the problem.

lldb/packages/Python/lldbsuite/test/concurrent_base.py

@@ -290,6 +290,10 @@ def do_thread_actions(
                         if funcname in f.GetDisplayFunctionName():
                             thread_has_user_code = True
                             break
+
+                    if thread_has_user_code:
+                        break
+
                 if thread_has_user_code:
                     num_threads_with_usercode += 1