Nightly tests failing because of legacy definitions update

[phackstock]

Since the update IAMconsortium/legacy-definitions@bad8c0c, the test that pulls from common- and legacy-definitions has been failing. The reason for that is that it tested for the presence of the variable Employment in the legacy-definitions. With the above commit it was moved.
To me the most obvious fix is to use the already existing pin feature for common- and legacy-definitions rather than adjust the variables we test for.

FYI @danielhuppmann