Add a test suite for the Image class

[OverloadedOrama]

Part of #43440. I created a test suite for the Image class. This is my first time ever writing unit tests (and my second PR to Godot), so feedback is appreciated! The tests cover:

• Image creation (create(), create_from_data(), copy_from())
• Saving images to .png and .exr files.
• Loading images from files (load() and load_x_from_buffer() methods, based on the work of @Calinou)
• Getters, such as get_data(), get_size(), get_width(), get_height(), get_format(), get_used_rect(), get_rect(), get_pixel() and get_pixelv().
• Image resizing with crop(), resize(), shrink_x2() and resize_to_po2().
• Modifying the image's pixels with set_pixel(), set_pixelv(), fill(), blend_rect(), blit_rect(), flip_x() and flip_y().
• Others, such as is_empty(), is_invisible(), is_compressed(), has_mipmaps().

The test suit is incomplete, the methods missing include most notably compression and mipmaps-related methods, bumpmap_to_normalmap(), srgb_to_linear() and some others, because I am not entirely sure how they work as I haven't used them so far.

[Xrayez]

These type of messages are redundant in my opinion ("X expected to be Y"). I'd just use CHECK() macro for those. My rule of thumb is: if I start to write rationale which contains the "expected" word, I just replace CHECK_MESSAGE to CHECK. 🙂

[OverloadedOrama]

All right, I changed some of the checks to just CHECK().

[akien-mga]

Thanks!