[lldb][AArch64] Fix flakiness in TestSVEThreadedDynamic

[DavidSpickett]

Summary

This test runs to a breakpoint on thread 0. Thread 0 then starts
thread 2 and 3, which both have breakpoints in them.

In https://lab.llvm.org/buildbot/#/builders/96/builds/41674
I think that we managed to do the first check on thread 2 before
thread 3 had started. Therefore "thread continue 3" failed.

So wait for all three to startup before we check their status.

I considered putting a timeout on the while like the wait_for... methods,
but the test itself already has a global timeout. Plus, I'd rather
not be tuning a timeout per piece of hardware this runs on.

99% of the time we will already have 3 threads when the check is done.

[DavidSpickett]

My brain says this is not a good way to ensure a state, but I couldn't think of anything that wouldn't have its own race conditions.

[omjavaid]

Sounds ok to me. We can always fix it if starts to cause trouble.

lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/TestSVEThreadedDynamic.py

def test_sve_registers_dynamic_config(self):
 
         thY_break_line2 = line_number("main.c", "// Thread Y breakpoint 2")
         lldbutil.run_break_set_by_file_and_line(self, "main.c", thY_break_line2)
 
         self.runCmd("run", RUN_SUCCEEDED)
 
         process = self.dbg.GetSelectedTarget().GetProcess()
 
-        thread1 = process.GetThreadAtIndex(0)
-
         self.expect(
             "thread info 1",
             STOPPED_DUE_TO_BREAKPOINT,
             substrs=["stop reason = breakpoint"],
         )
 
         if 8 in supported_vg:
             self.check_sve_registers(8)
         else:
             self.check_sve_registers(4)
 
         self.runCmd("process continue", RUN_SUCCEEDED)
 
+        # If we start the checks too quickly, thread 3 may not have started.
+        while (process.GetNumThreads() < 3):
+            pass
+
         for idx in range(1, process.GetNumThreads()):
             thread = process.GetThreadAtIndex(idx)
             if thread.GetStopReason() != lldb.eStopReasonBreakpoint:
                 self.runCmd("thread continue %d" % (idx + 1))
                 self.assertEqual(thread.GetStopReason(), lldb.eStopReasonBreakpoint)
 
             stopped_at_line_number = thread.GetFrameAtIndex(0).GetLineEntry().GetLine()