tests: Make ovsdb-server add-db/remove-db test faster and more reliable.

tests: Make ovsdb-server add-db/remove-db test faster and more reliable.

Until now, the "ovsdb-server/add-db and remove-db with --monitor" test
killed ovsdb-server with SIGSEGV twice.  Each time, the "--monitor" option
caused the supervisor process to restart the child, but the second time it
incurred a 10-second delay intended to prevent the daemon from wasting CPU
time by restarting itself and dying again very quickly in a loop.  This
made the test take over 10 seconds to execute.  It also made it
occasionally fail because the OVS_WAIT_UNTIL check waits at most
approximately 10 seconds before it decides that the condition that it is
testing for will never occur.

This commit fixes the problem by breaking the test into two tests, each of
which kills ovsdb-server with SIGSEGV only once.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>