TESTS/MICROBENCHMARKS: sources and makefiles are not installed

* What: currently the whole `tests/` directory is copied to the
installation path, instead of that only the binaries, definition files
and run script are installed for both tests and microbenchmarks
* When: default installation
* By: previous synchronizations from `$srcdir/tests/` to
`$prefix/tests` are removed and replaced by a cleaner code doing use
of default `autoconf` directives
* But: --