test: improve coverage on LPM tbl8

test: improve coverage on LPM tbl8

Existing test cases create 256 tbl8 groups for testing. The number covers
only 8 bit next_hop/group field. Since the next_hop/group field had been
extended to 24-bits, creating more than 256 groups in tests can improve
the coverage.

Coverage was not expanded to reach the max supported group number, because
it would take too much time to run for this fast-test.

Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Tested-by: David Christensen <drc@linux.vnet.ibm.com>
Acked-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>