test: use utf-8 rather than unicode literal

tests/cell.cpp

@@ -11,7 +11,7 @@ TEST_CASE("MultibyteWidth") {
   CHECK(1 == mbswidth(" "));      // single byte, one column
   CHECK(5 == mbswidth("abcde"));  // single byte, one column
   CHECK(1 == mbswidth("µ"));      // two bytes, one column
-  CHECK(2 == mbswidth("🦲"));     // four bytes, two columns
+  CHECK(2 == mbswidth("\xf0\x9f\xa6\xb2"));     // four bytes, two columns
   CHECK(6 == mbswidth("平仮名")); // nine bytes, six columns
   CHECK(1 == mbswidth("\ufdfd")); // three bytes, ? columns, wcwidth() returns 1
 }