Stabilize testQosSaiPgSharedWatermark by increasing the wartermark range

[JibinBao]

Description of PR

When run test of testQosSaiPgSharedWatermark, we meet the following error:

"stderr_lines": [ 

E                   "ptftests/remote.py:42: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.", 

E                   "  constants = yaml.load(fd)", 

E                   "sai_qos_tests.PGSharedWatermarkTest ... router_mac: 1c:34:da:bf:b7:00", 

E                   "dst_port_id: 0, src_port_id: 1 src_port_vlan: None", 

E                   "actual dst_port_id: 0", 

E                   "Init pkts num sent: 0, min: 0, actual watermark value to start: 144", 

E                   "pkts num to send: 3, total pkts: 9, pg shared: 161514", 

E                   "lower bound: 1296, actual value: 1296, upper bound (+2): 1584", 

E                   "pkts num to send: 13459, total pkts: 40386, pg shared: 161514", 

E                   "lower bound: 5815584, actual value: 5815584, upper bound (+2): 5815872", 

E                   "pkts num to send: 13459, total pkts: 80763, pg shared: 161514", 

E                   "lower bound: 11629872, actual value: 11629872, upper bound (+2): 11630160", 

E                   "pkts num to send: 13459, total pkts: 121140, pg shared: 161514", 

E                   "lower bound: 17444160, actual value: 17444160, upper bound (+2): 17444448", 

E                   "pkts num to send: 13458, total pkts: 161514, pg shared: 161514", 

E                   "lower bound: 23258016, actual value: 23258016, upper bound (+2): 23258304", 

E                   "exceeded pkts num sent: 13459, expected watermark: 23258448, actual value: 23258880", 

E                   "FAIL", 

E                   "", 

E                   "======================================================================", 

E                   "FAIL: sai_qos_tests.PGSharedWatermarkTest", 

E                   "----------------------------------------------------------------------", 

E                   "Traceback (most recent call last):", 

E                   "  File \"saitests/sai_qos_tests.py\", line 1906, in runTest", 

E                   "    assert(pg_shared_wm_res[pg] <= (expected_wm + margin + cell_occupancy) * cell_size)", 

E                   "AssertionError", 

E                   "", 

E                   "----------------------------------------------------------------------", 

E                   "Ran 1 test in 77.146s", 

E                   "", 

E                   "FAILED (failures=1)", 

E                   "No handlers could be found for logger \"dataplane\"" 

E               ], 

In order to stabilize the testQosSaiPgSharedWatermark, according to the experience value, increase margin from 2 to 3.
Summary:
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Back port request

• 201911

Approach

What is the motivation for this PR?

Stabilize the testQosSaiPgSharedWatermark.

How did you do it?

According to the experience value, increase margin from 2 to 3.

How did you verify/test it?

run the test of testQosSaiPgSharedWatermark

Any platform specific information?

no

Supported testbed topology if it's a new test case?

ptf32

Documentation