Remove assumption from py-symbol.exp

The current py-symbol.exp test makes an assumption about which symbol
will be returned first.  I don't think gdb should really make promises
about the order in which the symbols are listed, though, and a series
I am working on changes this behavior.  This patch changes the test to
merely ensure that both symbols are returned.

Approved-By: Simon Marchi <simon.marchi@efficios.com>