The font for this test case contains a ‘kern’ table of format 0, which in turns contains a single format 0 subtable. Without kerning, the glyphs for T, u and the dotless ı will look like normal letters. However, if your implementation correctly applies the font’s kerning data, the shapes will join to form a geometric pattern. If you can still discern the original letters ıTuTuTı in the rendering, it’s a sign that your implementation fails to correctly handle kerning. See also HarfBuzz bug 1255.