Improve test coverage. #99

Job	Run time
posix (Collect coverage special_fun, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, special_...	16m 39s
posix (Collect coverage distribution_tests, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, d...	7m 19s
posix (Collect coverage mp, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, mp)	3m 18s
posix (Collect coverage misc, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, misc)	9m 34s
posix (Collect coverage quadrature, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, quadrature)	15m 23s
posix (Collect coverage interpolators, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, interp...	28m 41s
posix (Collect coverage autodiff, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, autodiff)	4m 12s
	1h 25m 6s

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