selftests: forwarding: Increase maximum deviation in multipath test

We sometimes observe failures in the test due to too large discrepancy
between the measured and expected ratios. For example:

TEST: ECMP                                                          [FAIL]
        Too large discrepancy between expected and measured ratios
        INFO: Expected ratio 1.00 Measured ratio 1.11

Fix this by allowing an up to 15% deviation between both ratios.

Another possibility is to increase the number of generated flows, but
this will prolong the execution time of the test, which is already quite
high.

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