doc: add document for testing with seastar #479

Job	Run time
Test with C++20 modules enabled / test	11m 6s
Test (clang++, C++20, dev) / test	4m 58s
Test with DPDK enabled / test	11m 44s
Test (clang++, C++20, release) / test	5m 0s
Test (clang++, C++20, debug) / test	7m 11s
Test (clang++, C++23, dev) / test	4m 24s
Test (clang++, C++23, release) / test	16m 38s
Test (clang++, C++23, debug) / test	7m 15s
Test (g++, C++20, dev) / test	4m 24s
Test (g++, C++20, debug) / test	7m 33s
Test (g++, C++20, release) / test	7m 14s
Test (g++, C++23, debug) / test	8m 4s
Test (g++, C++23, dev) / test	4m 22s
Test (g++, C++23, release) / test	8m 23s
	1h 48m 16s

Provide feedback