gdb/testsuite: fix inferior-clone.exp for native-extended-gdbserver

003aae07 (gdb: Copy inferior properties
in clone-inferior) introduced a testcase that fails when testing with
the native-extended-gdbserver board:

    Running ../gdb/testsuite/gdb.base/inferior-clone.exp ...
    FAIL: gdb.base/inferior-clone.exp: inferior 2: clone-inferior
    FAIL: gdb.base/inferior-clone.exp: inferior 3: clone-inferior

The error is as follows:

    clone-inferior
    [New inferior 2]
    Added inferior 2 on connection 1 (extended-remote localhost:2346)
    (gdb) FAIL: gdb.base/inferior-clone.exp: inferior 2: clone-inferior

This fails because the testcase only expect the 'Added inferior 2' part
of the message.  The 'on connection 1 [...]' part is unexpected.

Fix by adjusting the testcase to a account for the possible trailing
part of the message.

Tested on x86_64-linux with native-extende-gdbserver and unix boards.

Change-Id: Ie3d6f04c9ffe9cab1fbda8ddf4935ee09b858c7a