Add tests for special_values in RasterPlotConfig

[claire-simpson]

Description

• Add raster_utils tests for SpecialValueConfig
• Fix deprecation warnings associated with matplotlib colormap objects being made immutable (e.g., use cmap=cmap.set_extremes instead of cmap.set_under) - issue noted in Reports: Matplotlib 3.11.0 compatibility #2632

Associated invest-test-data PR: https://bitbucket.org/natcap/invest-test-data/pull-requests/47

Fixes #2593

Checklist

• Updated HISTORY.rst and link to any relevant issue (if these changes are user-facing)
• Updated the user's guide (if needed)
• Tested the Workbench UI (if relevant)

[claire-simpson]

Tests failing due to #2632