Skip ECN config test on unsupported platform

[bingwang-ms]

Description of PR

Summary:
Fixes #8648
If is_valid_platform_and_version returns False, we will see below exception

**Description**

        try:
            if is_valid_platform_and_version(duthost, "WRED_PROFILE", "ECN tuning"):
                output = apply_patch(duthost, json_data=json_patch, dest_file=tmpfile)
                expect_op_success(duthost, output)
                ensure_application_of_updated_config(duthost, configdb_field, ",".join(values))
            else:
>               expect_op_failure(output)
E               UnboundLocalError: local variable 'output' referenced before assignment

It's because the output variable is not defined.
Actually, the test case should be skipped if the platform is unsupported.

Type of change

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

Back port request

• 201911
• 202012
• 202205

Approach

What is the motivation for this PR?

This PR is to skip ECN config test on unsupported platform.

How did you do it?

If is_valid_platform_and_version returns False, skip the test.

How did you verify/test it?

Verified on a SN4600 testbed

collected 4 items                                                                                                                                                                           

generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_min_threshold] SKIPPED                                                                        [ 25%]
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_max_threshold] SKIPPED                                                                        [ 50%]
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_drop_probability] SKIPPED                                                                     [ 75%]
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_min_threshold,green_max_threshold,green_drop_probability] SKIPPED                             [100%]

Any platform specific information?

No.

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

Not a new test case.

Documentation

[bingwang-ms]

Discussed with @isabelmsft , PR #8341 will be reverted from 202205 branch.