test

Add CI env for C++ #550

Job	Run time
check-misc (head)	6s
steep-check (head)	14s
test-ruby (head, master)	5m 27s
test-memory (head)	27s
test-ruby (3.0, master)	5m 37s
test-windows (head)	2m 3s
ruby-versions / Generate Ruby versions	2s
test-cpp (head, g++)	14s
test-cpp (head, clang++)	19s
test (2.5)	26s
test (2.6)	22s
test (2.7)	23s
test (3.0)	13s
test (3.1)	12s
test (3.2)	11s
test (3.3)	22s
test (head)	10s
	16m 48s

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