learn.at: Fix a shell syntax

Fix a botch in commit 7f12bdcc.
("learn.at: Fix a race in "self-modifying flow with idle_timeout" test")

Acked-by: Ben Pfaff <blp@nicira.com>
ofproto-dpif.at: Fix some races

These races were exposed on my environment by commit c2a77f33.
("tests/ofproto-dpif: Use vlog to test dpif behaviour.")
ofproto-dpif.at: Use OVS_APP_EXIT_AND_WAIT where appropriate

Leftover of commit 0c473314.
("ofproto-dpif.at: Wait for the monitor's pidfile disappears where necessary")
ofproto-dpif.at: Fix races in dpif/dump-flows test