I claim the ornamental design for a bathtub tray, as shown and described.
FIG. 1 is a perspective view from the top and front of a bathtub tray showing the new design;
FIG. 2 is a perspective view thereof from the top and rear;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a front view thereof; and,
FIG. 8 is a rear view thereof.
In the drawings, the broken lines shown in the figures depict unclaimed subject matter and/or environmental structure, and form no part of the claimed design.