tests/lwip: stabalize test script

While running this test script, I encountered the problem, that the
process for the previous test was not finished, not allowing the next
test to use its TAP interface. This small timer should fix that.

tests/lwip/tests/01-run.py

@@ -181,6 +181,8 @@ class TestStrategy(ApplicationStrategy):
                 test_case(board_group, self.app_dir, env=None)
                 sys.stdout.write('.')
                 sys.stdout.flush()
+                # wait a bit for tear down of test case
+                time.sleep(.2)
             print()