r.buffer: Added test script

[Shreshth-Malik]

This PR adds regression tests for the r.buffer module to ensure its functionality remains intact with future code changes. The following test cases are covered:

test_buffer_creation(): Verifies the creation of buffer zones around raster features based on specified distances. It checks that the output raster exists, has expected category values starting from 1, and that the minimum and maximum values match the expected range.

test_no_non_null_values(): Confirms that the r.buffer module correctly handles an input raster that is entirely null, ensuring the output contains no non-null values.

test_ignore_zero_values(): Ensures that zero values in the input raster are ignored when creating buffer zones, resulting in an output that does not include any zero categories.