Fix selective tests checks when system tests are modified

[potiuk]

When providers system tests were modified in provider tests were modified, selective checks had two bugs that compounded led to provider tests being skipped.

1. Modification of system tests lead to "all providers affected"
   condition (wrongly) - because "TESTS" were checked as root path
   before "SYSTEM TESTS" - and since system tests were subfolder
   of tests, the code that checked if system tests path belongs
   to provider never found it.

2. "All Providers affected" in such case (when it was not caused by
   another condition lead to "skip-provider-tests" set to true due
   to missing "else" in one of the conditions.

This PR fixes both cases and simplifies the conditions so that they are easier to understand and modify. Those conditions will be further simplified when we separate providers to separate projects #44511 because there we will not have to check separately several sub-folders of airflow, but for now it's good enough.

Some of those conditions are simplified as well because we are in Python 3.9+ and "is_relative_to" is now available in Path.

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[gopidesupavan]

This is really good fix, thanks Jarek.

It seems there are might be some pr's until now went out without providers tests. and good thing we have schedule run to spot out these like the one recent :)

[potiuk]

It seems there are might be some pr's until now went out without providers tests. and good thing we have schedule run to spot out these like the one recent :)

Well. I think it was quite rare - It's only when system tests were modified and NO provider.yaml was modified - that would trigger another path I think, and this is a rare case where system tests are modified together with regular tests and code. When you add new classes with system tests you also usually modify the provider.yaml to add those hooks/operators you added.

But yes. "canary" builds are necessary and they are running all tests by-design to catch such cases. That was an important and deliberate design decision.