tests: set exec program, disable clone

To support running the tests under valgrind, add two environment
variables:

    ALCOVE_TEST_EXEC : set the {exec, string()} parameter to
                       alcove_drv:start/1.

    ALCOVE_TEST_USE_FORK : use fork() instead of clone()

For example:

    ALCOVE_TEST_EXEC="sudo valgrind --leak-check=yes --log-file=alcove.log" \
        ALCOVE_TEST_USE_FORK=1 make test