Add an integration test for consistency with common-definitions

[danielhuppmann]

Remove GDP variables following the merging of IAMconsortium/common-definitions#54 and add an integration-test to ensure consistency between common-definitions & legacy definitions.

[danielhuppmann]

@phackstock, one minor issue in the test is that nomenclature.yaml is not deleted if the test fails. I tried briefly playing around with some try-except-finally but it become a bit complicated... And there isn't really any harm from that file not being deleted when running the test locally.

[phackstock]

Looks good to me.
Regarding the test not deleting the config file, using a try... finally in the test should solve the issue. I've put a solution that should work as suggestions.
Then you could also remove the entry from the .gitignore file.

[danielhuppmann]

Seems to work as expected, thanks for the hint!

[phackstock]

Now the entry in the .gitignore is no longer needed.
Other than that good to merge 👍