This PR introduces a comprehensive suite of tests for the i.fft GRASS GIS module. These tests cover a wide range of scenarios, including basic functionality, advanced properties like energy conservation and linearity, and edge cases such as all-zero and all-one rasters. Performance for larger datasets is also assessed.

Key updates in this PR include:

• Validation of FFT output existence and dimensions.
• Tests for linearity, energy conservation, and reconstruction via i.ifft.
• Coverage of edge cases for specific raster patterns.
• Performance assessment with large rasters.

Test Case Additions:

1. Basic Functionality:

   • Verify that i.fft produces real and imaginary output rasters.
   • Ensure output dimensions match the input raster.

2. Advanced Properties:

   • Linearity: Combine two rasters and compare FFT results to individual components.
   • Energy Conservation: Validate the energy conservation theorem.
   • Reconstruction: Test i.ifft for accurate reconstruction of the original raster.

3. Edge Cases:

   • Behavior with an all-zero raster.
   • Behavior with an all-one raster.

4. Performance:

   • Test FFT performance on a large 1000x1000 raster.