Fix gdb.python/py-finish-breakpoint.exp ERROR.

This patch fixes the following error, when running gdb.python/
py-finish-breakpoint.exp:

    ERROR: can't read "gdb_py_is_py24": no such variable
        while executing
    "if { $gdb_py_is_py24 == 1 } {

gdb/testsuite/ChangeLog:

        * gdb.python/py-finish-breakpoint.exp: Add skip_python_tests
        check.  Move $gdb_py_is_py24 value check right after.