test-execute: drop capabilities when testing with user manager

test-execute: drop capabilities when testing with user manager

Before this, tests are split into two categories, system and user, but
both are running in fully privileged environment. Hence, unprivileged
user scope was mostly not covered by the test.

Let's run all tests in both system and user scopes, and drop capabilities
when Manager is running in user scope.

This also makes the host environment protected more from the test run.