build: do not include -DBoost_TEST_DYN_LINK in seastar_testing_cflags #339

Job	Run time
Test with DPDK enabled / test	11m 25s
Test (clang++, C++20, debug) / test	6m 58s
Test (clang++, C++20, release) / test	4m 49s
Test (clang++, C++23, debug) / test	6m 52s
Test (clang++, C++20, dev) / test	4m 23s
Test (clang++, C++23, dev) / test	4m 34s
Test (clang++, C++23, release) / test	5m 8s
Test with C++20 modules enabled / test	11m 35s
Test (g++, C++20, debug) / test	7m 8s
Test (g++, C++20, dev) / test	4m 19s
Test (g++, C++20, release) / test	5m 22s
Test (g++, C++23, debug) / test	7m 19s
Test (g++, C++23, dev) / test	4m 16s
Test (g++, C++23, release) / test	5m 5s
	1h 29m 13s

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