doc: mention CORRADE_TESTSUITE_TEST_TARGET in building docs.

It's inherited from Corrade so nothing is explicitly needed here to
support it, but it's useful to mention it.

doc/building.dox

@@ -816,6 +816,12 @@ ctest -R GLTest # run only tests requiring OpenGL context
 Tests requiring Vulkan to work are also disabled by default, enable them with
 `MAGNUM_BUILD_VK_TESTS`.
 
+By default the tests are compiled as part of the implicit `ALL` target. Use the
+`CORRADE_TESTSUITE_TEST_TARGET` CMake variable to create a dedicated target for
+building just the tests. See the documentation of the
+@ref corrade-cmake-add-test "corrade_add_test()" CMake macro for more
+information.
+
 @section building-doc Building documentation
 
 The documentation is generated using [Doxygen](http://doxygen.org) with the