Fix global variable assignment in JS test suite
Fix global variable assignment in JS test suite

In test_imgproc.js, the test_filter suite's last test assigns a variable
to `size` without declaring it with `let`, polluting the global scope.
This commit adds `let` to the statement, so that the variable is scoped
to the test block.

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv