Fix reader test.
CMake config: Fix unit tests.

They were not actually run due to an error in the add_test() command.
Also this adds the test group (directory) name to tests so that there will be
no problems if we have tests with the same name in more than one group.
Split up OSMIUM_LIBRARIES into useful subsets.

Now available OSMIUM_PBF_LIBRARIES and OSMIUM_XML_LIBRARIES for the libraries
needed for reading and writing PBF and XML files, respectively. Or together
OSMIUM_IO_LIBRARIES. OSMIUM_LIBRARIES still holds all libraries Osmium will
ever need, but that shouldn't be used usually, because not all those libraries
are needed for all programs using libosmium.
Mixed cleanups for CMake config.
CMake config: Fix warnings settings for Dev mode.
Clean up data tests cmake config.
Remove memcpy from wkb code.
Use std::copy_n/fill_n instead of low-level memcpy/memset.

The compiler should do the right thing and use memcpy/memset or some other
efficient implementation internally.
Make settings of build types a bit more sensible.