tests/sys/kern: Skip capsicum procdesc tests when capability mode is unavailable

pdwait's capsicum/enotcap and procdesc's pdopenpid_capmode enter capability mode.
Require security_capability_mode (and security_capabilities for enotcap) so the
cases skip cleanly on kernels built without CAPABILITIES instead of failing.

Approved by: asomers, gallatin
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D58545