Heads-up: make check no longer available in default cmake config

[dmnks]

We have flipped the ENABLE_TESTSUITE cmake option's default value from ON to OFF (via #3391).

That means, if you find yourself wondering why the make check and make ci targets have suddenly ceased to exist in your newly created build directories, please note you need to enable them explicitly with -DENABLE_TESTSUITE=ON when configuring.