Revisit GTest library usage

[Muxianesty]

Currently, with every clean compilation (when the previous build directory is deleted) GTest source files are downloaded from Github, which requires Internet connection.

It would be better to be able to just install all the required dependencies with the Internet connection and then be able to compile the project without it.

At this point there are two ways of fixing this:

• Test-binary compilation is optional and has to be turned on explicitly
• Download libgtest-dev targets with the GTest source files and then pass them to CMake's target_link_libraries, but this might require knowing their exact directory of installation.

[Muxianesty]

Additionally, we need to utilize the latest release version of GTest, not a random commit.

[Muxianesty]

Closed until further notice.