Simplify the capsicum-test wrapper script

Instead of running tests one-by-one with the shell wrapper we now run
the full gtest testsuite twice (once as root, once as non root). This
significantly speeds up running tests despite running them twice.
This change also passes the missing -u flag to capsicum-test that caused
test failures (https://bugs.freebsd.org/250178)

Previously, running the testsuite with the wrapper script took ~3s per
test on aarch64 QEMU, i.e. a total of almost 5 minutes.
Now it takes 6 seconds to run all tests twice.

Before:
root@freebsd-aarch64:/usr/tests/sys/capsicum # /usr/bin/time kyua test functional
94/96 passed (2 failed)

309.97 real        58.46 user       244.31 sys

After:
root@freebsd-aarch64:/usr/tests/sys/capsicum # /usr/bin/time kyua test functional
functional:test_root -> passed [2.659s]
functional:test_unprivileged -> passed [2.391s]
2/2 passed (0 failed)

5.48 real         1.06 user         2.52 sys

This overhead is caused by kyua + atf-sh spawning lots of additional
processes and can be avoided by just running the googletest test binary.
syscall seconds calls errors
fork 39.810229456 1275 0
sigprocmask 13.546928736 572 0

i.e. 1275 processes spawned to run a single test.

Test Plan: All tests pass with D28907.
PR: 250178
Reviewed By: lwhsu
Differential Revision: https://reviews.freebsd.org/D29014