[Thinkit] Add a test case for sFlow backoff. Add a util function to verify dscp value.

[VSuryaprasad-HCL]

Key_word Check:

~/pins_upstream/sonic-buildimage/src/sonic-p4rt/sonic-pins/tests/sflow$ ~/tools/keyword_checks.sh .
Keyword check Passed.

Build result:

/sonic/src/sonic-p4rt/sonic-pins$ bazel build $BAZEL_BUILD_OPTS ...
INFO: Analyzed 636 targets (0 packages loaded, 0 targets configured).
INFO: Found 636 targets...
INFO: Elapsed time: 0.444s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action

Test result:

/sonic/src/sonic-p4rt/sonic-pins$ bazel test $BAZEL_BUILD_OPTS --cache_test_results=no ...
INFO: Analyzed 636 targets (0 packages loaded, 0 targets configured).
INFO: Found 424 targets and 212 test targets...
INFO: Elapsed time: 161.726s, Critical Path: 116.14s
INFO: 266 processes: 321 linux-sandbox, 18 local.
INFO: Build completed successfully, 266 total actions
//dvaas:port_id_map_test                                                 PASSED in 1.8s
//dvaas:test_run_validation_golden_test                                  PASSED in 0.1s
//dvaas:test_run_validation_test                                         PASSED in 1.3s
//dvaas:test_run_validation_test_runner                                  PASSED in 0.1s
//thinkit:mock_control_device_test                                       PASSED in 0.9s
//thinkit:mock_generic_testbed_test                                      PASSED in 1.0s
//thinkit:mock_mirror_testbed_test                                       PASSED in 1.5s
//thinkit:mock_ssh_client_test                                           PASSED in 0.2s
//thinkit:mock_switch_test                                               PASSED in 1.6s
//thinkit:mock_test_environment_test                                     PASSED in 0.1s
//thinkit:switch_test                                                    PASSED in 1.4s
//sai_p4/instantiations/google/tests:p4_constraints_integration_test     PASSED in 1.7s
  Stats over 5 runs: max = 1.7s, min = 1.4s, avg = 1.5s, dev = 0.1s
//sai_p4/instantiations/google/test_tools:table_entry_generator_test     PASSED in 44.0s
  Stats over 50 runs: max = 44.0s, min = 1.2s, avg = 3.2s, dev = 8.3s

Executed 212 out of 212 tests: 212 tests pass.
INFO: Build completed successfully, 266 total actions

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[jaanah-hcl]

@kishanps Kindly review the PR
Thanks

[mssonicbld]

/azp run

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