Cherry-pick #17075 to 7.x: Add support of TEST_TAGS in python tests

[jsoriano]

Cherry-pick of PR #17075 to 7.x branch. Original message:

What does this PR do?

Add a tag(tag) decorator that skips a test if the tag is not included
in the comma-separated list of TEST_TAGS environment variable.
This offers an initial support for the similar implementation added for
mage in #16937.

Why is it important?

#16937 was intended for cloud tests, where we only have go-based
integration tests. But we have found similar needings in some services
like oracle, where we need an image that we cannot distribute. In that
case we need a similar approach to run the test only in our Jenkins
instances, where the image is available.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works

How to test this PR locally

• Tag a python test with @metricbeat.tag('sometag').
• Check that the test is skipped if TEST_TAGS is not used.
• Check that the test is executed if TEST_TAGS environment variable includes sometag (e.g: TEST_TAGS=sometag or TEST_TAGS=sometag,foo.

Related issues

• Required by [Metricbeat] Make Oracle module GA #16833 to enable tests only in Jenkins.
• Extends Add build tags for cloud tests #16937.
• Could be useful in Add support for integration testing of cloud features #15891 if we add system tests for cloud features.

Use cases

• Run certain tests that require some application or credentials to be available only where they are.

[kaiyan-sheng]

Do we need a changelog in the developer changelog for this? The backport looks good.

[jsoriano]

Do we need a changelog in the developer changelog for this?

Good point, changelog entry added in #17140.