[HarfBuzz] Regression test for thai (0.9.5 vs 0.9.14)

[datao zhang]

Hi all: Font file: angsa.ttf, refer to attachment. After compared with two version, the dotted circle disappear when thai tone mark or dependent vowel displayed as single character. I think that's better to display the dotted circle when the base character lost(consonant character in thai).  Following list tone mark and vowel in thai: 

 
 
  Ñ
 
 
  Ô
 
 
  Õ
 
 
  Ö
 
 
  ×
 
 
  Ø
 
 
  Ù
 
 
  Ú
 
 
  í
 
 
  î
 
 
  ç
 
 
  è
 
 
  é
 
 
  ê
 
 
  ë
 
 
  ì
 

  Br,Dean 		 	   		  
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