seed database used for test suite maybe should live in ./target?

[davepacheco]

While debugging #540, I realized that I was running cargo test from two different clones of omicron with the same value of $TMPDIR in my environment, and that they could totally stomp on each other if one of their "build" steps initialized $TMPDIR/crdb-base while the other one's test suite was running trying to create a new database directory from that directory.

If we move the seed directory to ./target, it will at least be scoped to one local clone of Omicron.

[davepacheco]

Another reason: it's surprising that cargo clean doesn't remove the seed directory, since that's generated by the build.

This is obviated for now by #549 but eventually we'll probably want to re-add that and we'll want to address this.