Enable DTrace test suite by default

From: Li-Wen Hsu <lwhsu_at_freebsd.org>
Date: Thu, 24 Mar 2022 04:37:50 UTC
Hello,

With disabling the flaky test cases, the dtrace tests job looks stable now:

https://ci.freebsd.org/job/FreeBSD-main-amd64-dtrace_test/

I believe it's time to enable building and installing DTrace test
suite by default, to receive more tests and let us see issues earlier.

https://reviews.freebsd.org/D34656

Currently it's only enabled on amd64, but if you think it's reasonable
to also enable it on other architectures, please let me know. Thanks.

Best,
Li-Wen