imlib2: add y4m test examples

Prepared using:
```
$ ffmpeg -i test/images/icon-64.png -pix_fmt yuv420p test/images/icon-64.yuv420p.y4m
...
$ ffmpeg -i test/images/icon-64.png -pix_fmt yuv422p test/images/icon-64.yuv422p.y4m
...
$ ffmpeg -i test/images/icon-64.png -pix_fmt yuv444p test/images/icon-64.yuv444p.y4m
...
```

Tested:
(1) correct y4m files
```
$ LD_LIBRARY_PATH=~/proj/imlib2/src/lib/.libs/:${LD_LIBRARY_PATH} IMLIB2_MODULE_PATH=~/proj/imlib2/src/modules/loaders/.libs/ valgrind --leak-check=yes /bin/feh ~/proj/imlib2/test/images/icon-*y4m
==391567== Memcheck, a memory error detector
==391567== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==391567== Using Valgrind-3.20.0 and LibVEX; rerun with -h for copyright info
==391567== Command: /bin/feh /home/chemag/proj/imlib2/test/images/icon-64.yuv420p.y4m /home/chemag/proj/imlib2/test/images/icon-64.yuv422p.y4m /home/chemag/proj/imlib2/test/images/icon-64.yuv444p.y4m
==391567==
==391567==
==391567== HEAP SUMMARY:
==391567==     in use at exit: 297,719 bytes in 1,018 blocks
==391567==   total heap usage: 5,151 allocs, 4,133 frees, 871,023 bytes allocated
==391567==
==391567== LEAK SUMMARY:
==391567==    definitely lost: 0 bytes in 0 blocks
==391567==    indirectly lost: 0 bytes in 0 blocks
==391567==      possibly lost: 0 bytes in 0 blocks
==391567==    still reachable: 295,703 bytes in 997 blocks
==391567==         suppressed: 0 bytes in 0 blocks
==391567== Reachable blocks (those to which a pointer was found) are not shown.
==391567== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==391567==
==391567== For lists of detected and suppressed errors, rerun with: -s
==391567== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
```

(2) broken y4m file
Prepared by copying test/images/icon-64.yuv420p.y4m and replacing the "C420jpeg" item in the first line with "Cfoo" (invalid color space).

```
$ LD_LIBRARY_PATH=~/proj/imlib2/src/lib/.libs/:${LD_LIBRARY_PATH} IMLIB2_MODULE_PATH=~/proj/imlib2/src/modules/loaders/.libs/ valgrind --leak-check=yes /bin/feh ~/proj/imlib2/icon-64.yuv420p.y4m
==391699== Memcheck, a memory error detector
==391699== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==391699== Using Valgrind-3.20.0 and LibVEX; rerun with -h for copyright info
==391699== Command: /bin/feh /home/chemag/proj/imlib2/icon-64.yuv420p.y4m
==391699==
liby4m: Unrecognized colourspace foo
liby4m: Unable to read file's parameter list
liby4m: Failed to parse file header
***** Imlib2 Developer Warning ***** :
  This program is calling the Imlib call:

  imlib_image_set_changes_on_disk();

  With the parameter:

  image

  being NULL. Please fix your program.
***** Imlib2 Developer Warning ***** :
  This program is calling the Imlib call:

  imlib_image_get_width();

  With the parameter:

  image

  being NULL. Please fix your program.
***** Imlib2 Developer Warning ***** :
  This program is calling the Imlib call:

  imlib_image_get_height();

  With the parameter:

  image

  being NULL. Please fix your program.
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  1 (X_CreateWindow)
  Value in failed request:  0x0
  Serial number of failed request:  15
  Current serial number in output stream:  16
==391699==
==391699== HEAP SUMMARY:
==391699==     in use at exit: 303,880 bytes in 2,116 blocks
==391699==   total heap usage: 3,405 allocs, 1,289 frees, 670,313 bytes allocated
==391699==
==391699== LEAK SUMMARY:
==391699==    definitely lost: 0 bytes in 0 blocks
==391699==    indirectly lost: 0 bytes in 0 blocks
==391699==      possibly lost: 0 bytes in 0 blocks
==391699==    still reachable: 301,864 bytes in 2,095 blocks
==391699==         suppressed: 0 bytes in 0 blocks
==391699== Reachable blocks (those to which a pointer was found) are not shown.
==391699== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==391699==
==391699== For lists of detected and suppressed errors, rerun with: -s
==391699== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
```

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