The ornamental design for a test plug assembly, as shown and described.
FIG. 1 is a perspective view of a test plug assembly showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a front elevational view with the rear being a mirror image thereof;
FIG. 4 is a bottom view thereof; and,
FIG. 5 is a right side elevational view with the left side being a mirror image thereof.
The bracketed showing of the elongate threaded rods in FIGS. 3 and 5 indicates indefinte length. The broken line showing of a safety plug and plumbing hardware in FIGS. 1, 2, 3, and 5 is for illustrative purposes only and forms no part of the claimed design.