build: simplify test handling and speed up build.

build: simplify test handling and speed up build.

Instead of using a ./configure parameter to decide whehter to build
tests or not, use the check_PROGRAMS variable so that the tests are
only built when running `make check`.

To avoid slowing down the test phase itself, collapse the declaration
of the test targets in the tests/Makefile.am file, this way they can
be built and linked in parallel before the testsuite is executed.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>