Fix mock glyph removal now that check no longer mutates TTFont

The previous approach relied on the `glyphOrder` property being
populated as a side-effect of the "unreachable-glyphs" check, but this
no longer happens now that the accidental mutation in the check has been
removed.

This commit avoids using the internal property entirely by using the
`setGlyphOrder()` function, and ensuring that the font is fully loaded
prior to this to avoid the glyph being referenced during a later load
after it has already been removed from the order.

If this causes issues again, it may be more resilient for us to use
fontTools' subsetter library to implement this pytest.

(PR #4835)

tests/test_checks_universal.py

@@ -1053,15 +1053,20 @@ def test_check_unreachable_glyphs():
         assert glyph not in message
 
     ttFont = TTFont(TEST_FILE("notosansmath/NotoSansMath-Regular.ttf"))
+    ttFont.ensureDecompiled()  # (required for mock glyph removal below)
+    glyph_order = ttFont.getGlyphOrder()
+
     # upWhiteMediumTriangle is used as a component in circledTriangle,
     # since CFF does not have composites it became unused.
     # So that is a build tooling issue.
     message = assert_results_contain(check(ttFont), WARN, "unreachable-glyphs")
     assert "upWhiteMediumTriangle" in message
-    assert "upWhiteMediumTriangle" in ttFont.glyphOrder
+    assert "upWhiteMediumTriangle" in glyph_order
 
-    # Other than that problem, no other glyphs are unreachable:
-    ttFont.glyphOrder.remove("upWhiteMediumTriangle")
+    # Other than that problem, no other glyphs are unreachable;
+    # Remove the glyph and then try again.
+    glyph_order.remove("upWhiteMediumTriangle")
+    ttFont.setGlyphOrder(glyph_order)
     assert "upWhiteMediumTriangle" not in ttFont.glyphOrder
     assert_PASS(check(ttFont))