Add Unit tests about splitwebp

[ollelogdahl]

Hello. I want to contribute to this! :) Working on it now

[Razdeep]

Great!
GTest should be downloaded & built once if a flag like -DBUILD_TEST=ON is provided along with the CMake command.

[ollelogdahl]

Having some problems compiling the source. splitwebp.cpp requires fstream (std::fstream is defined in fstream). I will replace the iostream include for the fstream, since fstream itself includes iostream. Linux also requires hdl5 installed separately (dependency of libopencv_hdl), and therefore i have added it in the dependency list in README. I will add this in my final commit :)

[ollelogdahl]

Im still intrested in making actual tests for the project, if someone else has the experience to set up the correct environment.

[Razdeep]

Did you manage to compile the source on Linux? What distribution are you using? And what is the opencv version that you have downloaded?

For now, you can install GTest globally (using a package manager like apt-get), and write the tests. Later on, we'll figure out a way to use GTest as ExternalProject.

[ollelogdahl]

Using Archlinux. It compiled sucessfully, once i manually installed hdl5 and fixed the include error. Using openCV 4.4.
I will go ahead and write some tests, compiling them globally.