test: Keep player event streams open

test: Keep player event streams open

We're about to fix the server to properly notice when a player drops
their connection and then consider them inactive. To prepare for this
in the test suite, here we put the curl calls getting the event stream
into the background so they stick around, and finally kill them at the
end of the test suite.

We use an ugly `eval` based approach so that emapthy_player_activate
with a name like "alice" sets both an $alice_cookie and an $alice_pid
variable. (Sure would be nice if we had real data structures. Oh
well.)