i.albedo: Add test file

[codersmiti]

This PR introduces a test suite for the i.albedo GRASS GIS module, focusing on various scenarios such as sensor-specific albedo calculations, output range validation, aggressive mode processing, and linear reflectance scaling.

Key updates in this PR include:

Functionality:

• Verify albedo calculations for MODIS, Landsat, NOAA, and ASTER sensors using precomputed reference statistics.
• Ensure that computed albedo values are remaining within the 0 to 1 range.

Linearity Test and Landsat Aggressive Mode:

• Validate that aggressive mode processing reduces the dynamic range of output values compared to standard mode.
• Confirm that albedo calculations scale linearly with input reflectance values.

Test Case Additions:

• Sensor Tests: Validate albedo calculations for MODIS, Landsat, NOAA, and ASTER sensors against reference statistics.
• Aggressive Mode Comparison: Compare standard and aggressive processing modes to verify dynamic range reduction and mean shift.
• Output Range Validation: Ensure all albedo outputs are within the valid 0-1 range.
• Linearity Test: Confirm that doubling input reflectance values results in doubled albedo outputs.