Uses/dbus-testing.mk: Introduce new USES

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Summary

This USES starts a system- and session-level D-Bus daemons before the
test stage and stops them after, much like Uses/display.mk provides an
X11 display.

It is intended to be used with ports whose testing require a running dbus daemon.

Sponsored by: Future Crew, LLC

Test Plan

Doing USES+=dbus-testing for accessibility/at-spi2-core fixes
its make test. This USES also worked great when I was working in Future Crew
for our own dbus daemon ports.

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Now also required for ports-mgmt/packagekits tests.

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Ping.

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Ping.