cap_sysctl tests: Serialize

These tests fail spuriously when run in parallel, since some of them
write a value to a global sysctl, read it back, and expect to see the
written value. These tests complete quite quickly in any case.

MFC after: 1 week

(cherry picked from commit 5d96ca7a307279cf4bdb191b7074cd1efcd3b8e8)