The ornamental design for an diagnostic circuittestdevice, as shown and described.
FIG. 1 is a top, front, right side perspective view of the diagnostic circuit test device of the present invention;
FIG. 2 is a left side elevation view, the right side elevation view being a mirror image;
FIG. 3 is a front elevation view;
FIG. 4 is a rear elevation view;
FIG. 5 is a top plan view; and,
FIG. 6 is a bottom plan view.
The broken lines of the bottom surface shown in the drawings views are included for the purpose of illustrating environmental structure and form no part of the claimed invention.