TestDerivs: Add 6th order central finite difference #804

Job	Run time
download-build-test (cpu, real32, debug)	12m 17s
download-build-test (cpu, real64, debug)	14m 30s
download-build-test (cpu, real64, optimize)	23m 21s
download-build-test (cuda, real32, debug)	29m 3s
download-build-test (cuda, real64, debug)	27m 10s
download-build-test (cuda, real64, optimize)	33m 21s
download-build-test (rocm, real64, debug)	21m 28s
download-build-test (rocm, real64, optimize)	24m 19s
	3h 5m 29s

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