I claim the ornamental design for a seal for a test plug, as shown and described.
FIG. 1 is a perspective view from the front showing our design of a seal for a test plug.
FIG. 2 is a top view thereof showing our design of the seal for a test plug of FIG. 1, and wherein the bottom view is a mirror image; and,
FIG. 3 is a front view thereof showing our design of the seal for a test plug of FIG. 1, and wherein the rear side, left side, and right side views are mirror images of the front view.
The stippling and linear shading shown in the drawings represent a contrasting appearance.