fix test include directory, resolves #276 #327

Job	Run time
check-single-include	6s
test-ubuntu-20.04-gcc-9	1m 4s
test-ubuntu-20.04-gcc-11	1m 16s
test-ubuntu-20.04-clang-9	1m 31s
test-ubuntu-20.04-clang-12	1m 19s
test-ubuntu-22.04-clang-15-no-exceptions	1m 9s
test-ubuntu-22.04-gcc	49s
test-ubuntu-24.04-gcc	55s
test-windows-2019-msvc	2m 3s
test-windows-2019-clang	1m 43s
test-windows-2019-gcc	2m 24s
test-windows-2022-msvc	44s
test-macOS-12-gcc	1m 16s
test-macOS-13-clang	3m 2s
test-macOS-14-clang	1m 19s
	20m 40s

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