Add FreeBSD build and test using Cirrus-CI

Cirrus-CI is a hosted CI service that supports FreeBSD, Linux, macOS,
and Winodws.  Add a .cirrus.yml to provide CI coverage on pull requests
for FreeBSD 12.4 and 13.2.

.cirrus.yml

@@ -0,0 +1,19 @@
+freebsd_task:
+  freebsd_instance:
+    matrix:
+    - image: freebsd-13-2-release-amd64
+  sysctl_script:
+    - sysctl net.inet.tcp.blackhole=0
+    - sysctl net.inet.udp.blackhole=0
+  configure_script:
+    - mkdir build
+    - cd build
+    - ../configure
+  pythoninfo_script:
+    - cd build && make pythoninfo
+  build_script:
+    - cd build && make -j$(sysctl -n hw.ncpu)
+  test_script:
+    - cd build
+    # dtrace fails to build on FreeBSD - see gh-73263
+    - make buildbottest TESTOPTS="-j0 -x test_dtrace"

Misc/NEWS.d/next/Tests/2023-09-05-21-42-54.gh-issue-91960.abClTs.rst

@@ -0,0 +1 @@
+FreeBSD 13.2 CI coverage for pull requests is now provided by Cirrus-CI (a hosted CI service that supports Linux, macOS, Windows, and FreeBSD).