CMake: turn testing on, following KDE style.

A new option BUILD_TESTING is available; you can turn this off at
CMake-time, but building the tests themselves is harmless (although
it takes some CPU time).

The main advantage of doing this is that running the tests becomes
simple:
    make test
Instead of figuring out which tests there are and how to run them
individually. Note that the partition-manager tests will normally
fail, because they require an additional environment variable to
be set to tell them what disk to destroy.

CMakeLists.txt

@@ -59,6 +59,11 @@ option( WITH_PYTHON "Enable Python modules API (requires Boost.Python)." ON )
 option( WITH_CRASHREPORTER "Build with CrashReporter" ON )
 option( INSTALL_CONFIG "Install configuration files" ON)
 option( WITH_PYTHONQT "Enable next generation Python modules API (experimental, requires PythonQt)." OFF )
+option(BUILD_TESTING "Build the testing tree." ON)
+
+if(BUILD_TESTING)
+    enable_testing()
+endif ()
 
 if( CMAKE_SYSTEM_PROCESSOR MATCHES "arm" OR NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/thirdparty/libcrashreporter-qt/CMakeLists.txt" )
     message( STATUS "Build of crashreporter disabled." )