dev-libs/folks: don't require dbusmock without tests being built

dev-libs/folks: don't require dbusmock without tests being built

The upstream meson-0.53 fix made the dbusmock check actually effective,
erroring out if not found when bluetooth is enabled. We already have
a patch downstream to make build of tests conditional, so build on
top of that to skip the dbusmock check when tests aren't being built.

Fixes: 61a9ee210b2d44c4fa953342504293a89395bef0
Closes: https://bugs.gentoo.org/710548
Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Mart Raudsepp <leio@gentoo.org>