selftests: mlxsw: Make sampling test more robust

The test sometimes fails with an error message such as:

TEST: tc sample (w/ flower) rate (egress)                           [FAIL]
	Expected 100 packets, got 70 packets, which is -30% off. Required accuracy is +-25%

Make the test more robust by generating more packets, therefore
increasing the number of expected samples. Decrease the transmission
delay in order not to needlessly prolong the test.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>