It’s important to note that, differently from what we’ve seen for
    the serial test harness (see Parallel Test Harness), the
    AM_TESTS_ENVIRONMENT and TESTS_ENVIRONMENT variables cannot be use
    to define a custom test runner; the LOG_COMPILER and LOG_FLAGS (or
    their extension-specific counterparts) should be used instead:

    ## This is WRONG!
    AM_TESTS_ENVIRONMENT = PERL5LIB='$(srcdir)/lib' $(PERL) -Mstrict -w

    ## Do this instead.
    AM_TESTS_ENVIRONMENT = PERL5LIB='$(srcdir)/lib'; export PERL5LIB;
    LOG_COMPILER = $(PERL)
    AM_LOG_FLAGS = -Mstrict -w

(http://www.gnu.org/software/automake/manual/html_node/Parallel-Test-Harness.html)