Explicitly specify suitable font for combining-marks-position test
The test case checks for correct rendering of combining marks above the text,
instead of for example extending the run length by rendering them as a separate
glyph. For the purpose of a robust test case, a font should be chosen that has
all the required glyphs. Times New Roman on Linux - which is default selected
otherwise - does have a glyph for Heh but does not have a glyph for the Hamza
Before grapheme cluster font selection, Heh is rendered from Times New Roman,
with a Hamza glyph from DejaVu Sans above - which is not optimal. After grapheme
cluster based font selection, Heh and Hamza are combined from DejaVu Sans,
however, rendered as an isolated glyph separate from the rest of the run, which
is not optimal either.
More discussion on what is the real fix, e.g. selecting DejaVu Sans for the
whole run is in the bug.