man: add 'testing' as one of the suggestions for DEPLOYMENT=

Looking at the list, "test" or "testing" seems to be a fairly generic entry
that is missing from the list of suggestions. I went with "testing" because it
fits better with the other item, e.g. "staging".

In https://github.com/systemd/systemd/issues/38743 "laboratory" was also
suggested. I didn't include this because that is more about the location, not
deployment type. Any of the other deployments could be in a "laboratory".

Closes https://github.com/systemd/systemd/issues/38743.

man/machine-info.xml

@@ -120,6 +120,7 @@
           <literal>development</literal>,
           <literal>integration</literal>,
           <literal>staging</literal>,
+          <literal>testing</literal>,
           <literal>production</literal>.
           </para>