tests: Fix time dependency in overlapping flows modification test. #2268

Job	Run time
linux rpm fedora	6m 29s
dpdk gcc	9s
osx clang --disable-ssl	6m 28s
linux deb no dpdk	8m 2s
linux gcc --disable-ssl	4m 6s
linux clang --disable-ssl	3m 4s
linux gcc c99	3m 33s
linux clang c99	3m 47s
linux gcc test	12m 2s
linux clang test asan	19m 52s
linux clang test ubsan	14m 21s
linux gcc test --enable-shared	14m 51s
linux clang test --enable-shared	17m 44s
linux gcc test dpdk	14m 25s
linux clang test dpdk	10m 53s
linux gcc test -ljemalloc	10m 57s
linux clang test -ljemalloc	13m 14s
linux gcc --enable-afxdp	3m 56s
linux clang --enable-afxdp	4m 7s
linux gcc dpdk --enable-shared	5m 48s
linux clang dpdk --enable-shared	4m 1s
linux gcc dpdk-shared	3m 33s
linux clang dpdk-shared	4m 19s
linux gcc dpdk-shared --enable-shared	5m 45s
linux clang dpdk-shared --enable-shared	4m 16s
linux gcc m32 --disable-ssl	4m 31s
	3h 24m 13s

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