test-execute: Add tests for new PassEnvironment= directive

test-execute: Add tests for new PassEnvironment= directive

Check the base case, plus erasing the list, listing the same variable
name more than once and when variables are absent from the manager
execution environment.

Confirmed that `sudo ./test-execute` passes and that modifying the test
cases (or the values of the set variables in test-execute.c) is enough
to make the test cases fail.