The ornamental design for a testinginstrument, as shown and described.
FIG. 1 is a front, right-side, top perspective view of the testing device of the design according to a second embodiment;
FIG. 2 is a rear, left-side, top perspective view of thereof;
FIG. 3 is a partial front elevational view thereof;
FIG. 4 is a partial rear elevational view thereof;
FIG. 5 is a partial top plan view thereof;
FIG. 6 is an enlargement of area 6 in FIG. 1; and,
FIG. 7 is an enlargement of area 7 in FIG. 2.
The broken lines depict either (i) enlargement areas that form no part of the claimed design, or (ii) parts of the testing device that form no part of the claimed design.