[gdb/testsuite] Handle older python in gdb.python/py-send-packet.py

[gdb/testsuite] Handle older python in gdb.python/py-send-packet.py

With python 3.4, I run into:
...
Traceback (most recent call last):^M
  File "<string>", line 1, in <module>^M
  File
  "outputs/gdb.python/py-send-packet/py-send-packet.py", line 128, in \
    run_set_global_var_test^M
    res = conn.send_packet(b"X%x,4:\x02\x02\x02\x02" % addr)^M
TypeError: Could not convert Python object: b'X%x,4:\x02\x02\x02\x02'.^M
Error while executing Python code.^M
...
while with python 3.6 this works fine.

The type of addr is <class 'gdb.Value'>, so the first thing to try is whether
changing it into a string works:
...
    addr_str = "%x" % addr
    res = conn.send_packet(b"X%s,4:\x02\x02\x02\x02" % addr_str)
...
which gets us the more detailed:
...
TypeError: unsupported operand type(s) for %: 'bytes' and 'str'
...

Fix this by avoiding the '%' operator in the byte literal, and use instead:
...
def xpacket_header (addr):
    return ("X%x,4:" % addr).encode('ascii')
  ...
    res = conn.send_packet(xpacket_header(addr) + b"\x02\x02\x02\x02")
...

Tested on x86_64-linux, with python 3.4 and 3.6, and a backported version was
tested on the gdb-12-branch in combination with python 2.7.