[lldb][AArch64] Account for extra libc frames in PAC unwind test

Running this on Amazon Ubuntu the final backtrace is:
```
(lldb) thread backtrace
* thread #1, name = 'a.out', stop reason = breakpoint 1.1
  * frame #0: 0x0000aaaaaaaa07d0 a.out`func_c at main.c:10:3
    frame #1: 0x0000aaaaaaaa07c4 a.out`func_b at main.c:14:3
    frame #2: 0x0000aaaaaaaa07b4 a.out`func_a at main.c:18:3
    frame #3: 0x0000aaaaaaaa07a4 a.out`main(argc=<unavailable>, argv=<unavailable>) at main.c:22:3
    frame #4: 0x0000fffff7b373fc libc.so.6`___lldb_unnamed_symbol2962 + 108
    frame #5: 0x0000fffff7b374cc libc.so.6`__libc_start_main + 152
    frame #6: 0x0000aaaaaaaa06b0 a.out`_start + 48
```
This causes the test to fail because of the extra ___lldb_unnamed_symbol2962 frame
(an inlined function?).

To fix this, strictly check all the frames in main.c then for the rest
just check we find __libc_start_main and _start in that order regardless
of other frames in between.

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D154204

Author: DavidSpickett

lldb/test/API/functionalities/unwind/aarch64_unwind_pac/TestAArch64UnwindPAC.py

@@ -42,17 +42,31 @@ def test(self):
             "func_b",
             "func_a",
             "main",
+        ]
+
+        libc_backtrace = [
             "__libc_start_main",
             "_start",
         ]
-        self.assertEqual(thread.GetNumFrames(), len(backtrace))
-        for frame_idx, frame in enumerate(thread.frames):
-            frame = thread.GetFrameAtIndex(frame_idx)
+
+        self.assertTrue(thread.GetNumFrames() >= (len(backtrace) + len(libc_backtrace)))
+
+        # Strictly check frames that are in the test program's source.
+        for frame_idx, frame in enumerate(thread.frames[:len(backtrace)]):
             self.assertTrue(frame)
             self.assertEqual(frame.GetFunctionName(), backtrace[frame_idx])
-            # Check line number for functions in main.c
-            if frame_idx < 4:
-                self.assertEqual(
-                    frame.GetLineEntry().GetLine(),
-                    line_number("main.c", "Frame " + backtrace[frame_idx]),
-                )
+            self.assertEqual(
+                frame.GetLineEntry().GetLine(),
+                line_number("main.c", "Frame " + backtrace[frame_idx]),
+            )
+
+        # After the program comes some libc frames. The number varies by
+        # system, so ensure we have at least these two in this order,
+        # skipping frames in between.
+        start_idx = frame_idx + 1
+        for frame_idx, frame in enumerate(thread.frames[start_idx:], start=start_idx):
+            self.assertTrue(frame)
+            if libc_backtrace[0] == frame.GetFunctionName():
+                libc_backtrace.pop(0)
+
+        self.assertFalse(libc_backtrace, "Did not find expected libc frames.")