tests: Fix flaky "ovn-controller incremental processing". #3315

Job	Run time
osx clang --disable-ssl	2m 3s
linux rpm fedora	7m 5s
Prepare container	3m 13s
linux gcc --disable-ssl	2m 48s
linux clang --disable-ssl	2m 36s
linux gcc test -300	9m 26s
linux gcc test 301-600	6m 16s
linux gcc test 601- unstable	6m 24s
linux clang test sanitizers -300	12m 47s
linux clang test sanitizers 301-600	9m 10s
linux clang test sanitizers 601- unstable	10m 52s
linux gcc test -ljemalloc -300	9m 58s
linux gcc test -ljemalloc 301-600	6m 23s
linux gcc test -ljemalloc 601- unstable	7m 5s
linux gcc system-test-dpdk dpdk -100	16m 45s
linux gcc system-test-dpdk dpdk 101- unstable	14m 8s
linux gcc system-test-userspace -100	16m 21s
linux gcc system-test-userspace 101- unstable	12m 48s
linux gcc system-test -100	18m 32s
linux gcc system-test 101- unstable	17m 15s
linux clang system-test sanitizers -100	20m 9s
linux clang system-test sanitizers 101- unstable	19m 13s
linux x86 gcc --disable-ssl	2m 29s
	3h 53m 46s

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