build: don't rebuild everything for libtest.la

We added a -fvisibilty=default to the libtest.la CFLAGS. When automake
sees that the CFLAGS are different from those of libxkbcommon.la, it
figures it should recompile all of the $(libxkbcommon_la_SOURCES),
instead of reusing the already-compiled object files.

But actually, the convenience library is not a shared object, it's just
used as an archive, thus the -fvisibility is entirely pointless. So
avoid the recompilations by just removing it.

Signed-off-by: Ran Benita <ran234@gmail.com>
Move benchmarks from tests to their own files in bench/

The tests only contain tests, and the benchmarks are more visible.

Signed-off-by: Ran Benita <ran234@gmail.com>