CMake: support actually running benchmarks, as well as building them

At present it's possible (albeit clunky) to build benchmarks (yay!) using the new system, but there's no way to actually start a run of them. For example, ninja tst_bench_qqcollator builds the new benchmark I'm adding; it'd be nice if ninja tst_bench_qqcollator_benchmark would build it and run it.

Of course, I can run the benchmark by hand, once built. Then again, given that I used to need to make sure I had LD_LIBRARY_PATH and QT_PLUGIN_PATH set when building with qmake, I'm left wondering where the target_wrapper.sh has gone and whether I need to set those environment variables when running the benchmark. (Am I accidentally picking up the libraries and plugins from my default (unoptimised debug) build tree instead of the release build I carefully made to run the benchmarks from ? I guess not, as they're not set in my environment at all, but the doubt is still there.)

On the existing clunkiness: I have notes suggesting (although I can't see how the old configure script implemented them, so this may have gone away long ago) that -[no]make tests or -[no-]compile-tests controlled whether tests were built; having such options (and likewise for benchmarks) to configure would be more user-friendly than -DQT_BUILD_TESTS=ON, albeit perhaps this new form is good for getting us used to the way to do general tweaks to CMake at configure-time. It would also be nice if turning on benchmarks automagically turned on tests (it might do this already, but Alexandru advised me to do both explicitly when telling me how to turn on benchmarks, suggesting it doesn't).