The ornamental design for a scan tool, as shown and described.
FIG. 1 is a top view of the invention;
FIG. 2 is a front view of the invention;
FIG. 3 is a left side view of the invention;
FIG. 4 is a bottom view of the invention;
FIG. 5 is a right side view of the invention; and,
FIG. 6 is a hack view of the invention.
The broken line showing of indicia is included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.