[QoS/test_buffer] [Mellanox] Enhance test_buffer (#4350)

Signed-off-by: Kebo Liu <[email protected]>

Enhance the buffer test with:

* implemented the algorithm to calculate the headroom value based on the input port attributes
* for SONiC image version higher than 202106, compare the headroom against the calculated result based on the port attributes, for SONiC image version 202106 and before, keep the old way, comparing the values from pg_profile_lookup.ini

What is the motivation for this PR?
To make the test more accurate regarding the headroom comparison, on new SONiC versions(later than 202106), use the same algorithm implemented in sonic-swss to calculate the expected headroom values instead of look up the pg_profile_lookup.ini. The relevant SONiC headroom calculation change is described in sonic-net/SONiC#870

How did you do it?
* implement the same headroom calculating algorithm as in the sonic-swss.
* compare the headroom values against the values calculated by the above new algorithm fruition

How did you verify/test it?
Run buffer regression test on various Mellanox platform

Any platform specific information?
This change only impacts the Mellanox platform