Feature: define environment variables for CTest

[SSE4]

closes #3511

Changelog: Feature: define environment variables for CTest
Docs: conan-io/docs#1018

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^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

/cc @cjdb

[memsharded]

Looks good!

Please add it to the docs, and update the above https://github.com/conan-io/docs/pull/XXXX with the matching PR and I will merge this for next release. Thanks!

[SSE4]

will do soon

[cjdb]

Wonderful, thank you @SSE4!

[SSE4]

docs added