fix keyerror of PR sonic-net#10382 (sonic-net#11722)

What is the motivation for this PR?
PR sonic-net#10382 caused below two KeyError:

    @pytest.fixture(scope="function", autouse=False)
    def skip_pacific_dst_asic(self, dutConfig):
>       if dutConfig['dstDutAsic'] == "pac":
E       KeyError: 'dstDutAsic'
        if "wm_pg_shared_lossless" in pgProfile:
            pktsNumFillShared = qosConfig[pgProfile]["pkts_num_trig_pfc"]
        elif "wm_pg_shared_lossy" in pgProfile:
>           if dutConfig['dstDutAsic'] == "pac":
E           KeyError: 'dstDutAsic'
and remove duplicated code in PR sonic-net#11553 and PR sonic-net#10838, it will cause pre-commit failure

    @pytest.fixture(scope="function", autouse=False)
    def skip_longlink(self, dutQosConfig):
        portSpeedCableLength = dutQosConfig["portSpeedCableLength"]
        match = re.search("_([0-9]*)m", portSpeedCableLength)
        if match and int(match.group(1)) > 2000:
            pytest.skip(
                "This test is skipped for longlink.")
        yield
        return
How did you do it?
fix keyerror and remove duplicated code

tests/qos/qos_sai_base.py

@@ -2155,7 +2155,7 @@ def _skip_watermark_multi_DUT(
 
     @pytest.fixture(scope="function", autouse=False)
     def skip_pacific_dst_asic(self, dutConfig):
-        if dutConfig['dstDutAsic'] == "pac":
+        if dutConfig.get('dstDutAsic', 'UnknownDstDutAsic') == "pac":
             pytest.skip(
                 "This test is skipped since egress asic is cisco-8000 Q100.")
         yield

tests/qos/test_qos_sai.py

@@ -1566,7 +1566,7 @@ def testQosSaiPgSharedWatermark(
         if "wm_pg_shared_lossless" in pgProfile:
             pktsNumFillShared = qosConfig[pgProfile]["pkts_num_trig_pfc"]
         elif "wm_pg_shared_lossy" in pgProfile:
-            if dutConfig['dstDutAsic'] == "pac":
+            if dutConfig.get('dstDutAsic', 'UnknownDstDutAsic') == "pac":
                 pytest.skip(
                     "PGSharedWatermark: Lossy test is not applicable in "
                     "cisco-8000 Q100 platform.")