Import review changes

1. Collapse the declaration/assignment for trap_enotcap_enabled_len to reduce complexity.
2. Move the GTEST_SKIP() backwards compatible definition out into the global space.
3. Sort conditional platform #includes with FreeBSD before linux.

Requested by: David Drysdale <drysdale@google.com> [1,2], emaste [3]
Pull Request: https://github.com/google/capsicum-test/pull/42