Introduce layout test rendering a ligature composed of a space glyph

LayoutTests/third_party/AhemSpaceLigature/LICENCE.txt

Index: LayoutTests/third_party/AhemSpaceLigature/LICENCE.txt
diff --git a/LayoutTests/third_party/AhemSpaceLigature/LICENCE.txt b/LayoutTests/third_party/AhemSpaceLigature/LICENCE.txt
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+The AhemSpaceLigature font in this directory belongs to the public domain. In
+jurisdictions that do not recognize public domain ownership of these
+files, the following Creative Commons Zero declaration applies:
+
+<http://labs.creativecommons.org/licenses/zero-waive/1.0/us/legalcode>
+
+which is quoted below:
+
+  The person who has associated a work with this document (the "Work")
+  affirms that he or she (the "Affirmer") is the/an author or owner of
+  the Work. The Work may be any work of authorship, including a
+  database.
+
+  The Affirmer hereby fully, permanently and irrevocably waives and
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+  heirs or successors.
+
+  The Affirmer understands and intends that the Waiver has the effect
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