GDB: testsuite: Python: Don't return 0 from top-level (sed)

This patch is purely the result of running:

$ sed -i 's/^    return -1/    return/' *.exp*

inside gdb/testsuite/gdb.python.  A couple of false positives weren't
included.

Approved-By: Tom Tromey <tom@tromey.com>