py_test_usdt3 has shown flakiness recently, and I actually reproduced
it on FC27. This patch increases the iteration number for
kprobe_poll and I ran the test for more than 20 times
without any failure.

Also fix a python3 compability issue. Open the file to write with
attribute "wb" instead of "w" since the text string is a byte array.
Signed-off-by: Yonghong Song <yhs@fb.com>