Support `$WP_VERSION=trunk` or `$WP_VERSION=nightly` in WP test install
Support `$WP_VERSION=trunk` or `$WP_VERSION=nightly` in WP test install

While technically `trunk` is different than `nightly`, pragmatically
they're the same, and I'd like to use a built archive instead of SVN
checkout.

miya0001/wp-cli,GaryJones/wp-cli,gitlost/wp-cli,rodrigoprimo/wp-cli,rodrigoprimo/wp-cli,GaryJones/wp-cli,duncanjbrown/wp-cli,trepmal/wp-cli,aaemnnosttv/wp-cli,gitlost/wp-cli,shawnhooper/wp-cli,gitlost/wp-cli,2ndkauboy/wp-cli,GaryJones/wp-cli,shawnhooper/wp-cli,miya0001/wp-cli,rodrigoprimo/wp-cli,trepmal/wp-cli,aaemnnosttv/wp-cli,duncanjbrown/wp-cli,duncanjbrown/wp-cli,2ndkauboy/wp-cli,nyordanov/wp-cli,nyordanov/wp-cli,2ndkauboy/wp-cli,gedex/wp-cli,gedex/wp-cli,nyordanov/wp-cli,gedex/wp-cli,trepmal/wp-cli,miya0001/wp-cli,aaemnnosttv/wp-cli,shawnhooper/wp-cli