tests: Fix race in "ofproto-dpif - ofproto-dpif-monitor 1" test.

tests: Fix race in "ofproto-dpif - ofproto-dpif-monitor 1" test.

This test contained two commands that both read and overwrote
ovs-vswitchd.log, and then expected the running ovs-vswitchd to carry on
appending to it.   Depending on the shell implementation and the speed of
execution, and the libc implementation, this might not have the desired
effect.  This commit replaces this by a multi-step process that avoids
in-place replacement.

Found by inspection.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>