test_password_modify_non_utf8 should set default password storage scheme

[vashirov]

Issue Description
Test dirsrvtests/tests/suites/password/password_test.py::test_password_modify_non_utf8 assumes the default password storage scheme is PBKDF2_SHA256, but it's not true on old versions.
The test should set PBKDF2-SHA512 explicitly, since it tests a crash in pwdchan plugin that provides this storage scheme.