capsicum-test: Move out of contrib

Google developed the Capsicum unit test suite[1] as part of the
Capsicum-Linux[2] project, based on unit tests that existed in FreeBSD
and unit tests developed as

capsicum-test: Move out of contrib

Google developed the Capsicum unit test suite[1] as part of the
Capsicum-Linux[2] project, based on unit tests that existed in FreeBSD
and unit tests developed as part of the initial Capsicum-Linux port.

Capsicum-Linux was archived as of October 31, 2022 and is no longer
being maintained. FreeBSD is currently the only consumer of and
contributor to the test suite. Move the src into tests/sys/capsicum to
simplify ongoing maintenance.

The makefiles were deleted as we (continue to) use the existing bespoke
FreeBSD Makefile, and CONTRIBUTING.md was removed as the Google CLA is
no longer applicable.

[1] https://github.com/google/capsicum-test
[2] https://github.com/google/capsicum-linux

Reviewed by: asomers, oshogbo
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54917

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