Fix flake in TestZerothFrame.py

This test is relying on the order of `process.threads` which is
nondeterministic. By selecting the thread based on whether it is
stopped at our breakpoint we can reliably select the correct one.

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

@@ -40,28 +40,24 @@ def test(self):
         target = self.dbg.CreateTarget(exe)
         self.assertTrue(target, VALID_TARGET)
 
-        bp1_line = line_number("main.c", "// Set breakpoint 1 here")
-        bp2_line = line_number("main.c", "// Set breakpoint 2 here")
-
-        lldbutil.run_break_set_by_file_and_line(
-            self, "main.c", bp1_line, num_expected_locations=1
-        )
-        lldbutil.run_break_set_by_file_and_line(
-            self, "main.c", bp2_line, num_expected_locations=1
-        )
+        main_dot_c = lldb.SBFileSpec("main.c")
+        bp1 = target.BreakpointCreateBySourceRegex("// Set breakpoint 1 here", main_dot_c)
+        bp2 = target.BreakpointCreateBySourceRegex("// Set breakpoint 2 here", main_dot_c)
 
         process = target.LaunchSimple(None, None, self.get_process_working_directory())
         self.assertTrue(process, VALID_PROCESS)
 
-        thread = process.GetThreadAtIndex(0)
+        thread = self.thread()
+
         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(),
-            bp1_line,
+            thread.frame[0].GetLineEntry().GetLine(),
+            bp1.GetLocationAtIndex(0).GetAddress().GetLineEntry().GetLine(),
             "LLDB reported incorrect line number.",
         )
 
@@ -70,15 +66,14 @@ def test(self):
         # 'continue' command.
         process.Continue()
 
-        thread = process.GetThreadAtIndex(0)
         if self.TraceOn():
             print("Backtrace at the second breakpoint:")
             for f in thread.frames:
                 print(f)
         # Check that we have stopped at the breakpoint
         self.assertEqual(
             thread.frame[0].GetLineEntry().GetLine(),
-            bp2_line,
+            bp2.GetLocationAtIndex(0).GetAddress().GetLineEntry().GetLine(),
             "LLDB reported incorrect line number.",
         )
         # Double-check with GetPCAddress()