The ornamental design for a folding bathtub, as shown and described.
FIG. 1 is a front perspective view of the folding bath tub showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a left elevational view thereof;
FIG. 5 is a right elevational view thereof;
FIG. 6 is a front elevational view thereof; and,
FIG. 7 is a rear elevational view thereof.
The thin solid lines in the views have been provided for the purpose of illustrating the uniformity of the planar surfaces and the contour of the curved surfaces and are not a claimed surface pattern.
The structural features depicted by broken lines in the drawing have been provided for the purpose of illustrating portions that form no part of the claimed design.