test: add tests for incomplete UTF-8 sequences in codePointAt and remove TODO comment

Add test cases for single-byte incomplete two-octet and three-octet UTF-8 sequences
in code_point_at_incomplete.wren, and remove the resolved TODO comment about testing
truncated UTF-8 buffer reads in wrenUtf8Decode.

src/vm/wren_utils.c

@@ -143,7 +143,6 @@ int wrenUtf8Decode(const uint8_t* bytes, uint32_t length)
   }
 
   // Don't read past the end of the buffer on truncated UTF-8.
-  // TODO: Test this.
   if (remainingBytes > length - 1) return -1;
 
   while (remainingBytes > 0)