CMake: Remove the dead cmake/tests directory

The tests were gated behind BUILD_CMAKE_TESTING, which defaults to OFF
and is not set anywhere in CI. They have not been runnable for a long
time either: both test projects include QtSetup or QtBuild standalone,
and QtBuild.cmake immediately calls
qt_internal_setup_build_and_global_variables, which is not defined in
that context.

test.cmake, main.cpp and empty.cpp are left over from the moc, uic and
rcc tests that were removed earlier; nothing references them. test.cmake
even fakes QT_MOCSCANNER, which no longer exists.

The two remaining test projects have been ported to
tests/auto/cmake/RunCMake.

Change-Id: Ice486a09e40c6636b463fcc4c0c022c5926fc59f
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>