test: atomicity.py: improve exit probability on failure

Some gdb python exceptions on some os environments (e.g. macOS
Sierra, non-codesigned gdb) do not make gdb exit (but to drop down
to nonexistent command line?).
Mitigate this chance by explict SystemExit on all exceptions.
The contents of output file 'gdb.out' is unchanged.

test/atomicity.py

@@ -71,4 +71,8 @@ class RenameBreakpoint(gdb.Breakpoint):
         return False
 RenameBreakpoint('rename')
 
+try:
     gdb.execute('run')
+except Exception:
+    import traceback
+    raise SystemExit(traceback.format_exc())