improvements on tests of imagecodecs module

[sturkmen72]

[WIP] trivial improvements on tests of imagecodecs module

[sturkmen72]

i was not intending to prepare a PR to merge. but now i thing the proposed changes will be useful.

please review and consider the file changes baboon.ppm and read.png ( let me create a PR to opencv_extra if this PR acceptable)

[asmorkalov]

@sturkmen72 What is your motivation of png -> ppm replacement?

[sturkmen72]

@sturkmen72 What is your motivation of png -> ppm replacement?

1. i replaced baboon.ppm with correct one ( older was an unrelated image and probably not used in tests) using this new image in tests will be meaningful.
2. "#if defined(HAVE_PNG) || defined(HAVE_SPNG)" alredy exist in code and HAVE_PNG can be OFF in very rare cases.
3. a bit related "IT’S TIME TO RETIRE LENA FROM COMPUTER SCIENCE"

As a final word, the proposed changes are open to discussion. It may be unimportant. Let me do required changes according devs opinions.

[asmorkalov]

@mshabunin @opencv-alalek what is you opinion?

[mshabunin]

We have GDAL library integration which should support all these image types (PNG, JPEG, TIFF). Perhaps it should be accounted too, though I didn't try to test GDAL backend specifically (with no other backends).

[vpisarev]

I would choose beautiful Lena over less-than-beautiful baboon. Honestly, I don't see any real justification to make this change.

[asmorkalov]

👍