test

Add CI env for C++ and fix errors and warnings when compiling as C++ #1375

Job	Run time
check-misc (head)	11s
steep-check (head)	19s
test-ruby (head, master)	6m 27s
test-memory (head)	27s
test-ruby (3.0, master)	6m 16s
test-windows (head)	1m 39s
ruby-versions / Generate Ruby versions	3s
test-cpp (head, g++)	15s
test (2.5)	22s
test-cpp (head, clang++)	16s
test (2.6)	25s
test (2.7)	24s
test (3.0)	22s
test (3.1)	13s
test (3.2)	11s
test (3.3)	14s
test (head)	12s
	18m 16s

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