Initial version of integration tests

[iboB]

This is the initial version of the integration test framework and several tests:

• Basics - Check CMake cache entries after configuring based on environment variables and cmake command-line arguments. Incomplete. Many more invariants to go.
• Simple - Configure with a package, reconfigure with another version, reconfigure with option off. Complete (almost?). More assertions might possibly be added
• Source Cache - With source cache enabled configure a project with multiple deps, reconfigure by adding another, check cache directory. Close to complete. More checks can likely be added

This is obviously a very minimal (to say the least) collection of integration tests, but my proposal is that we merge this as it is and then discuss what new tests to make and perhaps have more users join in in writing them.

I suspect that as we write more and more tests, the framework will change dramatically but what we have in this PR seems, at least to me, as a decent starting point.

Some points of interest:

• I have added some documentation. It can be browsed in human-readable form here
• Integration tests are included in the CI test script and they pass. However I changed the platform windows-latest to windows-2022 (later than latest 😄). The thing is that windows-latest comes with ruby 2.5. This is a pretty old version and it cannot execute the tests as they are. Currently the effort port to 2.5 is minimal, but it may cause more problems in the future. The tests as they are can be executed with Ruby 2.7.0 or later and I propose that we support whatever is available in ubuntu-latest. (Installing a modern version of Ruby on windows-latest is of course possible, but I opted for the simplest decision).
• Because of the previous point the checks for this PR will not pass. They expect tests on windows-latest to pass, but there is no such job now.
• I updated the Format dependency of the style tests to the latest version so I could exclude the integration tests from this check. The templates contain invalid CMake code.

[TheLartians]

Awesome, thanks for the PR! Looks great on first look, but let me take a closer look in the next days when I have more time.

[TheLartians]

I think we could also use the official ruby/setup-ruby action instead, which is a little more clear in its intent, and requires no explanation.

      - uses: ruby/setup-ruby@v1
        with:
          ruby-version: 2.7

But I'm also fine with keeping windows-2022.

[iboB]

Actually I didn't realize setting up Ruby would be that easy. I still have the travis/appveyor mindset where installing different versions of software requires complex arcane platform-specific scripts, but of course on github there should be a one-liner for that.

I'm now in favor of keeping windows-latest, as it's always suspicious (at least to me) when a single concrete version of an OS is added in the CI :)

[iboB]

Well... I was too quick to praise the setup-ruby action. The problem is that it adds mingw in the path and this confuses CMake. We could still work around this, but not over-complicating the CI scripts is a better idea IMO.

So I'm reverting my commit and keeping windows-2022

[TheLartians]

Darn, I also would have expected it to just work, but then I agree that windows-2022 is the way to go!

[TheLartians]

Great work on the documentation and testing library, this looks like a great start for the integration tests! Looking forward to extend on this in the future.