The ornamental design for a diagnostic teststick, as shown and described.
FIG. 1 is a front elevation view of a diagnostic test stick in accordance with the present design;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a front perspective view thereof; and,
FIG. 8 is a rear perspective view thereof.
Portions of the diagnostic test stick shown by evenly-spaced dashed lines in FIGS. 1-8 depict environmental structures and represent unclaimed portions of the diagnostic test stick. The short-dash, long-dash line around the circumference of the diagnostic test stick in FIGS. 1, 3, and 5-8 is a boundary line defining the bounds of the claimed design and forms no part thereof.