Remove the support to build each test on its own

[BenjaminW3]

Currently each test can be build completely separate from all other tests. Each test is a new project which searches for alpaka, boost and the test common library and links them. They can be copied out of the alpaka repo and be built from anywhere as long as the ALPAKA_ROOT variable is correctly set.
This makes the CMakeLists unnecessarily complex.
The tests CMakeLists could be nearly half the size if we would only search for dependencies and apply test settings in the top-level test folders CMakeLists.txt. This is also the de-facto standard for CMake projects. I do not see a reason to extract single tests from the repo. The tests will always be built in combination with the alpaka library itself.
The examples currently have the same features. However, there I can see a reason why one would want to build them independently. Therefore I would keep everything as is for the examples.
What do you think?

[ax3l]

yes, tests don't need a full project, .cmake snippets are enought

Ideally tests should just be build with the root CMakeLists.txt as long as the CMake commonly standardized variable BUILD_TESTING is ON. This is what #476 added as interface (also #478 keeps this).

Instead of adding a add_subdirectory for each test, just include() them as .cmake files that define targets that use dependencies from the root CMakeLists.txt (or a find_package(... CONFIG) of alpaka). #478 tries to unify this further, but did not yet change the include of the tests away from add_subdirectory.