emulators/yuzu: don't run tests by default on beefy6

Not reproducible in a local 12.0 amd64 poudriere jail:

<phase: stage >

> Staging for yuzu-qt5-s20190702

[0/1] cd /wrkdirs/usr/ports/emulators/yuzu-qt5/work/.build && /usr/local/bin/ctest --force-new-ctest-process
Test project /wrkdirs/usr/ports/emulators/yuzu-qt5/work/.build

Start 1: tests

>> Killing runaway build after 7200 seconds with no output

Reported by: pkg-fallout