The ornamental design for a rack for specimencontainers, as shown and described.
FIG. 1 is a perspective view of a rack for specimen containers, showing the new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The dash lines in the drawings depict portions of the rack for specimen containers that form no part of the claimed design. The dot dash broken lines depict the boundaries of the claim and form no part thereof.