test: fix root check for test-bpf-foreign-programs

test: fix root check for test-bpf-foreign-programs

This test requires root, but the check was mistakenly dropped, causing it
to fail with an assert when running without root:

src/test/test-bpf-foreign-programs.c:308: Assertion failed: expected "test_bpf_cgroup_programs(m, "single_prog.service", single_prog, ELEMENTSOF(single_prog))" to succeed, but got error: Operation not permitted

Restore the uid check

Follow-up for 22e2f0642897cfa7ba975527f5394bd7fcdf639b