Fix flake in TestZerothFrame.py

This test is relying on the order of `process.threads` which is
nondeterministic. By switching from `process.GetThreadAtIndex` to
`process.GetThreadByIndex` we consistently retrieve the correct thread.

Author: kendal

lldb/test/API/functionalities/unwind/zeroth_frame/TestZerothFrame.py

@@ -53,14 +53,15 @@ def test(self):
         process = target.LaunchSimple(None, None, self.get_process_working_directory())
         self.assertTrue(process, VALID_PROCESS)
 
-        thread = process.GetThreadAtIndex(0)
+        thread = process.GetThreadByIndexID(1)
         if self.TraceOn():
             print("Backtrace at the first breakpoint:")
             for f in thread.frames:
                 print(f)
+
         # Check that we have stopped at correct breakpoint.
         self.assertEqual(
-            process.GetThreadAtIndex(0).frame[0].GetLineEntry().GetLine(),
+            thread.frame[0].GetLineEntry().GetLine(),
             bp1_line,
             "LLDB reported incorrect line number.",
         )
@@ -70,7 +71,7 @@ def test(self):
         # 'continue' command.
         process.Continue()
 
-        thread = process.GetThreadAtIndex(0)
+        thread = process.GetThreadByIndexID(1)
         if self.TraceOn():
             print("Backtrace at the second breakpoint:")
             for f in thread.frames: