The ornamental design for the diagnostic test strip, as shown and described.
FIG. 1 is a perspective view of a first embodiment of our new diagnostic test strip design;
FIG. 2 is an a front elevation view thereof;
FIG. 3 is a left-side elevation view thereof (the right-side elevation view being identical); and
FIG. 4 is a top plan view thereof (the bottom plan view being identical).
FIG. 5 is a perspective view of a second embodiment of our new diagnostic test strip design, wherein solid black surface shading represents contrasting color;
FIG. 6 is an a front elevation view thereof;
FIG. 7 is a left-side elevation view thereof (the right-side elevation view being identical); and,
FIG. 8 is a top plan view thereof (the bottom plan view being identical).
The broken lines in the drawing figures are included for the purpose of illustrating portions of the diagnostic test strip that form no part of the claimed design