Bug 1664969 - Disable async font list initialization under editor/libeditor/tests r=m_kato,jfkthame

According to the debug log and result on tryserver, the frequency of this
test failure becomes higher is caused by bug 1676966.  When global reflow
is kicked during dragging the grabber, the implicit capture is released
forcibly.  Fortunately, editor behavior shouldn't depend on font list
loading system, so, disabling the feature until fixing bug 1688430 is fine.

Differential Revision: https://phabricator.services.mozilla.com/D102889

editor/libeditor/tests/mochitest.ini

@@ -2,6 +2,7 @@
 prefs =
   editor.white_space_normalization.blink_compatible=true
   apz.zoom-to-focused-input.enabled=false
+  gfx.font_rendering.fallback.async=false
 support-files =
   data/cfhtml-chromium.txt
   data/cfhtml-firefox.txt