test/eal: check number of cores before running subtests

The eal flags unit test assumes that a certain number of cores are
available (4 and 8 cores), however this may not always be the case.
Individual developers may run the unit test on their local desktop
which typically have 2 to 4 cores, in said case the test is bound
to fail for lacking 4 or 8 cores.

Additionally, as we push forward introducing CI into DPDK we are limited
to the hardware specification of CI services (e.g. Travis CI) that only
have 2 cores on their servers, in which case the test would fail.

To fix this we check available cores before running a subtest. This
applies to subtests that are dedicated to test that the -l and --lcore
flags work correctly. If not enough cores are available the subtest is
simply skipped, otherwise the subtest is run.

Signed-off-by: Michael Santana <msantana@redhat.com>
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>