[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.

gdb/testsuite/gdb.python/py-send-packet.py

@@ -114,6 +114,10 @@ def check_global_var(expected_val):
     if val != expected_val:
         raise gdb.GdbError("global_var is 0x%x, expected 0x%x" % (val, expected_val))
 
+# Return a bytes object representing an 'X' packet header with
+# address ADDR.
+def xpacket_header (addr):
+    return ("X%x,4:" % addr).encode('ascii')
 
 # Set the 'X' packet to the remote target to set a global variable.
 # Checks that we can send byte values.
@@ -125,7 +129,7 @@ def run_set_global_var_test():
     res = conn.send_packet("X%x,4:\x01\x01\x01\x01" % addr)
     assert isinstance(res, bytes)
     check_global_var(0x01010101)
-    res = conn.send_packet(b"X%x,4:\x02\x02\x02\x02" % addr)
+    res = conn.send_packet(xpacket_header(addr) + b"\x02\x02\x02\x02")
     assert isinstance(res, bytes)
     check_global_var(0x02020202)
 
@@ -142,7 +146,7 @@ def run_set_global_var_test():
     assert saw_error
     check_global_var(0x02020202)
     # Now we pass a bytes object, which will work.
-    res = conn.send_packet(b"X%x,4:\xff\xff\xff\xff" % addr)
+    res = conn.send_packet(xpacket_header(addr) + b"\xff\xff\xff\xff")
     check_global_var(0xFFFFFFFF)
 
     print("set global_var test passed")