SF: Clean up RefreshRateConfigs tests

Finish renaming `config` to `mode`, and reduce repetition in preparation
of RefreshRateConfigs::RefreshRate removal.

Bug: 185535769
Bug: 129481165
Test: libsurfaceflinger_unittest
Change-Id: I9a87cec76387edbdfaeb31c1f24cff341489e45a
2 files changed