Fix test of warning filters in gpython.
The tests for gpython's handling of warning filters assumed that
the warnings passed in the command line were located on the top
of the warning filters list.
This is not true in the presence of automatically imported modules
that set warning filters, such as _distutils_hack, which
used to filter deprecation warnings from distutils.
We fix it by comparing against a regex which allows extra filters above or below the ones we set.