Add an option to disable unittest build, & disable them on Docker build

[nerzhul]

All is in the title.
It's not always useful to build unittests, especially on the docker build and on release builds (maybe).
Add a switch for that

To do

This PR is Ready for Review.

How to test

build the project with -DBUILD_UNITTESTS=0 and see that the unittest flag disappear from our command line

[SmallJoker]

Opting out unittests for faster compiling and lighter binaries. I get the idea, but IMO those tests can also be used as basic sanity checks which should be possible to run at all times when in doubt.

As long the unittests are compiled into Minetest by default for all configurations, I might accept this proposal if you'd add an user feedback like so:

	// Run unit tests
	if (cmd_args.getFlag("run-unittests")) {
#if BUILD_UNITTESTS
		return run_tests();
#else
		errorstream << "Unittests are not compiled into this binary."
			<< " Set the CMake parameter BUILD_UNITTESTS=1" << std::endl;
		return 1;
#endif
	}

[sfan5]

Well there are certain platforms where you can't even run unittests, Android is one of them and also currently builds without them.

[nerzhul]

docker build has no interest to contain the unittest, it's a base image you just run. I don't see the point to add a command line option saying hey compile with unittests if you want to run it, unittests are for devel purposes

[nerzhul]

are we fine ?

[nerzhul]

PR has been updated to:

• show an error message and keep the flag
• disable client unittests too (forgotten in first version)

[SmallJoker]

LGTM