Integrate capsicum-test into the FreeBSD test suite

This change takes capsicum-test from upstream and applies some local changes to make the
tests work on FreeBSD when executed via Kyua.

The local

Integrate capsicum-test into the FreeBSD test suite

This change takes capsicum-test from upstream and applies some local changes to make the
tests work on FreeBSD when executed via Kyua.

The local modifications are as follows:
1. Make `OpenatTest.WithFlag` pass with the new dot-dot lookup behavior in FreeBSD 12.x+.
2. capsicum-test references a set of helper binaries: `mini-me`, `mini-me.noexec`, and
`mini-me.setuid`, as part of the execve/fexecve tests, via execve, fexecve, and open.
It achieves this upstream by assuming `mini-me*` is in the current directory, however,
in order for Kyua to execute `capsicum-test`, it needs to provide a full path to
`mini-me*`. In order to achieve this, I made `capsicum-test` cache the executable's
path from argv[0] in main(..) and use the cached value to compute the path to
`mini-me*` as part of the execve/fexecve testcases.
3. The capsicum-test test suite assumes that it's always being run on CAPABILITIES enabled
kernels. However, there's a chance that the test will be run on a host without a
CAPABILITIES enabled kernel, so we must check for the support before running the tests.
The way to achieve this is to add the relevant `feature_present("security_capabilities")`
check to SetupEnvironment::SetUp() and skip the tests when the support is not available.
While here, add a check for `kern.trap_enotcap` being enabled. As noted by markj@ in
https://github.com/google/capsicum-test/issues/23, this sysctl being enabled can trigger
non-deterministic failures. Therefore, the tests should be skipped if this sysctl is
enabled.

All local changes have been submitted to the capsicum-test project
(https://github.com/google/capsicum-test) and are in various stages of review.
Please see the following pull requests for more details:
1. https://github.com/google/capsicum-test/pull/35
2. https://github.com/google/capsicum-test/pull/41
3. https://github.com/google/capsicum-test/pull/42

Reviewed by: asomers
Discussed with: emaste, markj
Approved by: emaste (mentor)
MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D19758

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