travis: do not run performance test in parallel under coverage
travis: do not run performance test in parallel under coverage

Performance tests have some limits to detect if they pass or fail
but under coverage execution is slow, running tests in parallel is
also very slow. Without that patch scheduler test simetimes fails.

Signed-off-by: Maxim Uvarov <db4d16e02ae2d7493db430203537da8b2e34f290@linaro.org>
Reviewed-by: Bill Fischofer <52f3c909d51cc5d355a68a403df6906b3c1a8f83@linaro.org>

dkrot/odp,dkrot/odp,dkrot/odp,dkrot/odp