tests: don't install test executables

tests: don't install test executables

I'm not sure what historical reasons there were to install test programs
if they are built but now I can't see any anymore. Tests can be run from
the build directory and users such as meta-openembedded can install them
and the relevant libraries (libgpiosim) manually into the filesystem as
they don't usually live in ${bindir} anyway. Installing test binaries just
pollutes the filesystem now so stop doing it.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>