TST: fix failing BMP tests on Windows

Fixes:
```
running test_bmp_reference ...
Tests\test_bmp_reference.py:85: assert_true(False, "Unsupported Image %s: %s" %(f,msg)) failed:
- Unsupported Image Tests/images/bmp/g\pal4rle.bmp: Unsupported BMP compression (2)
Tests\test_bmp_reference.py:85: assert_true(False, "Unsupported Image %s: %s" %(f,msg)) failed:
- Unsupported Image Tests/images/bmp/g\pal8rle.bmp: Unsupported BMP compression (1)
Tests\test_bmp_reference.py:85: assert_true(False, "Unsupported Image %s: %s" %(f,msg)) failed:
- Unsupported Image Tests/images/bmp/g\rgb32bf.bmp: Unsupported BMP bitfields layout
```