Added cmake option for disabling tests

[EvilBeaver]

It's useful to have an option for disabling tests when building on CI server.

Use Case:

• Imagine you have a project which uses cppkafka as a submodule
• You build project with CI server and for full-source build you do a git checkout with a recursive submodule update
• Now after checkout you have googletest submodule also cheсked out and cppkafla will be doing a tests build
• But as a submodule, cppkafka is always checked out on one specified commit and this means it's code is unchanged and stable. So it doesn't require any continuous testing.
• Having cppkafka tests built on your daily builds all you have is a time waste

This PR introduces CMake option CPPKAFKA_BUILD_TESTS which is enabled by default and old behavior is untouched. But it's easy to disable building tests with this option.

[mfontanini]

Thanks for this PR! The tests are not included in the all Makefile target so I don't think normally this would be an issue since the build will just skip them.

Anyway, this is a reasonable change and I can see its usefulness. The only thing I'd change is the option name. I normally prefer names in the form of <project>_DISABLE_<feature> for these types of parameters. Maybe CPPKAKFA_DISABLE_TESTS? Otherwise this looks like CPPKAFKA_BUILD_SHARED but has a very different meaning: the latter controls how something is built and the former controls what is being built.

On a similar note, there should probably be a similar flag to disable examples as well, but that can be tacked later.

[EvilBeaver]

Yes, i chose parameter name looking at CPPKAFKA_BUILD_SHARED option.
I build it using cmake and Visual Studio so Makefile and it's targets are not involved. Okay, I'll change option name

[EvilBeaver]

done

[mfontanini]

Fair enough, I didn't consider that. Looks like I typo-ed the option name and you copy pasted it. It says CPPKAKFA. Could you correct it? Sorry about that!

[EvilBeaver]

Copypastes are evil :) Fixed

[mfontanini]

lol yeah, also I should have probably worn my glasses while I replied in the morning. Thanks, I'll merge this later!