Match the working directory on remote host

Match the working directory on remote host

The test in gdb.python/python.exp tests "extended-prompt" and expects
working directory is printed.  However, working directory on remote
host doesn't have "gdb/testsuite", so the test fails on remote host
like:

set extended-prompt \w ^M
^M
/home/yao FAIL: gdb.python/python.exp: set extended prompt working directory (timeout)

This patch is to get the working directory first, and use it to match
the output of "set extended-prompt \\w ".  It works for remote host
and non remote host.

gdb/testsuite:

2014-11-02  Yao Qi  <yao@codesourcery.com>

	* gdb.python/python.exp: Get working directory and match the
	output of "set extended-prompt \\w " with it.