tests: run manager in session mode

False positives pop up otherwise.

FAIL: test-unit-name (exit: 134)
================================

Failed to open /dev/tty0: Permission denied
Failed to create root cgroup hierarchy: Permission denied
Assertion 'manager_new(SYSTEMD_SYSTEM, &m) == 0' failed at src/test/test-unit-name.c:125, function test_unit_printf(). Aborting.