More modules use this than I had expected!

Now the entire connection_t hierarchy is extracted from or.h

For once, it's a type that is used almost nowhere else besides the
logical place.

This one was actually fairly simple.

Also make sure that we're actually running the test from within the right
cwd, like we do when we're building.  This seems necessary to avoid
an error when running offline.

Amusingly, it appears that we had this bug before: we just weren't
noticing it, because of bug 26258.

Shouldn't send RELAY_COMMAND_DATA cell with 0 stream id.

"ours" to avoid version bump.