Fix checking stringifiability of containers with ostream operator<< o… #436

Job	Run time
test-linux-add_subdirectory	37s
test-linux-findpackage	24s
test-linux-fetchcontent	51s
test-macos-add_subdirectory	31s
test-macos-fetchcontent	34s
test-macos-findpackage	33s
test-mingw-add_subdirectory	2m 41s
test-mingw-fetchcontent	2m 41s
test-mingw-findpackage	2m 6s
test-windows-add_subdirectory	2m 1s
test-windows-fetchcontent	1m 59s
test-windows-findpackage	1m 25s
	16m 23s

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