Add testdata path in testing/helper.py
Add testdata path in testing/helper.py

This is to cover the case where TINK_SRC_PATH/testdata is present and should be used as testdata in non-bazel runs.

PiperOrigin-RevId: 456038760

google/tink,google/tink,google/tink,google/tink,google/tink,google/tink,google/tink,google/tink