tests: Fix time dependency in overlapping flows modification test. #1834

Job	Run time
linux gcc --disable-ssl	4m 12s
linux clang --disable-ssl	3m 2s
linux gcc c99	3m 6s
linux clang c99	2m 58s
linux gcc test	11m 49s
linux clang test asan	20m 44s
linux clang test ubsan	11m 40s
linux gcc test --enable-shared	14m 1s
linux clang test --enable-shared	14m 1s
linux gcc test dpdk	10m 53s
linux clang test dpdk	12m 39s
linux gcc test -ljemalloc	11m 59s
linux clang test -ljemalloc	10m 1s
linux gcc afxdp 5.3	4m 49s
linux clang afxdp 5.3	3m 59s
linux gcc dpdk --enable-shared	6m 53s
linux clang dpdk --enable-shared	5m 54s
linux gcc dpdk-shared	3m 36s
linux clang dpdk-shared	4m 21s
linux gcc dpdk-shared --enable-shared	5m 9s
linux clang dpdk-shared --enable-shared	4m 21s
linux gcc m32 --disable-ssl	4m 17s
osx clang --disable-ssl	3m 45s
linux deb no dpdk	8m 59s
linux deb shared dpdk	11m 5s
	3h 18m 13s

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