exercises(reverse-string): exclude three new tests (#589)

Exclude the new upstream tests [1]. For now, continue to allow solutions
that don't support unicode to work.

[1] exercism/problem-specifications@94cae6e938be

exercises/practice/reverse-string/.meta/tests.toml

@@ -26,3 +26,15 @@ description = "a palindrome"
 
 [b9e7dec1-c6df-40bd-9fa3-cd7ded010c4c]
 description = "an even-sized word"
+
+[1bed0f8a-13b0-4bd3-9d59-3d0593326fa2]
+description = "wide characters"
+include = false
+
+[93d7e1b8-f60f-4f3c-9559-4056e10d2ead]
+description = "grapheme cluster with pre-combined form"
+include = false
+
+[1028b2c1-6763-4459-8540-2da47ca512d9]
+description = "grapheme clusters"
+include = false