Improve reference tests

[fintelia]

This PR uses the libtest-mimic and walkdir crates to simplify/improve our reference tests. It recursively walks the entire directory tree under the reference images directory to find tests, and tests that cannot be run because of a disabled feature are now marked as "ignored" rather than silently not run.

I removed the "render_images" test because I didn't see what purpose it was serving that we didn't get by just having the normal tests write to tests/output/ with the observed contents of any image that didn't match the reference.

Also corrects several reference images because due to a bug in the old implementation we were apparently not testing the images under tests/reference/jpg...

Old output

running 2 tests
test check_references ... ok
test render_images ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 2.15s

New output

     Running tests/reference_images.rs (target/debug/deps/reference_images-6fce3ac7ddc64c75)

running 135 tests
test reference tests/images/bmp/images/Info_X1_R5_G5_B5.bmp                              ... ok
test reference tests/images/bmp/images/V3_R5_G6_B5.bmp                                   ... ok
test reference tests/images/bmp/images/V3_X8_R8_G8_B8.bmp                                ... ok
test reference tests/images/bmp/images/Info_8_Bit_Top_Down.bmp                           ... ok
test reference tests/images/bmp/images/V3_X4_R4_G4_B4_Top_Down.bmp                       ... ok
test reference tests/images/bmp/images/Info_A8_R8_G8_B8_Top_Down.bmp                     ... ok
test reference tests/images/bmp/images/Info_1_Bit.bmp                                    ... ok
test reference tests/images/bmp/images/V3_R5_G6_B5_Top_Down.bmp                          ... ok
test reference tests/images/hdr/images/rgbr4x4.hdr                                       ... ok
test reference tests/images/jpg/progressive/test.jpg                                     ... ok
test reference tests/images/bmp/images/pal8v4.bmp                                        ... ok
test reference tests/images/bmp/images/Info_1_Bit_Top_Down.bmp                           ... ok
[...]
test reference tests/images/png/apng/ball.png[15]                                        ... ok
test reference tests/images/png/apng/ball.png[1]                                         ... ok
test reference tests/images/png/apng/ball.png[13]                                        ... ok
test reference tests/images/png/apng/ball.png[7]                                         ... ok
test reference tests/images/png/apng/ball.png[17]                                        ... ok
test reference tests/images/png/apng/ball.png[8]                                         ... ok
test reference tests/images/webp/lossless_images/multi-color.webp                        ... ok
test reference tests/images/png/bugfixes/debug_triangle_corners_widescreen.png           ... ok
test reference tests/images/jpg/progressive/3.jpg                                        ... ok

test result: ok. 135 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.11s

[197g]

I like it except we still don't know how many tests to expect to be ran here. If the files are missing as from a crate then, if I read this correctly, silently no tests are generated or ran. Correct?

[fintelia]

Yes, though in the specific case of running from a .crate it won't matter because our Cargo.toml excludes everything under tests/ which means that tests/reference_images.rs isn't captured either...

The other case that isn't handled is files that are present under tests/images but not under tests/reference. There's currently a couple dozen such images that are used by other tests though, so we can't just treat them as failing tests. I suppose that listing them as ignored would be an option?

[197g]

Alright let's worry about packaging at a later time.