Refine test matrix generation. #423

Job	Run time
build (clang++, 2.7)	5s
build (3.9, g++, c++17)	1m 1s
build (2.7, g++, c++17)	1m 1s
build (3.14, g++, c++17)	59s
build (c++14, 2.7)	7s
build (clang++, teeks99/boost-python-test:clang-12_1.76.0)	1m 16s
build (3.12, g++, c++17)	1m 1s
build (3.14t, g++, c++17)	1m 3s
build (c++11, 2.7)	7s
build (clang++, 3.14t)	14s
build (c++98, 2.7)	4s
build (3.13, g++, c++17)	1m 0s
build (3.11, g++, c++17)	54s
build (3.1, g++, c++17)	1m 1s
	9m 53s

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