test-execute: Clarify interaction of PassEnvironment= and MANAGER_USER

@evverx brought up that test-execute runs under MANAGER_USER which
forwards all its environment variables to the services. It turns out it
only forwards those that were in the environment at the time of manager
creation, so this test was still working.

It was still possible to attack it by running something like:
  $ sudo VAR1=a VAR2=b VAR3=c ./test-execute

Prevent that attack by unsetting the three variables explicitly before
creating the manager for the test case.

Also add comments explaining the interactions with MANAGER_USER and,
while it has some caveats, this tests are still valid in that context.

Tested by checking that the test running with the variables set from the
external environment will still pass.

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