unicode: minor cleanup

These test cases are redundant because TestSimpleFold tests
all possible rotations of test data, so no need to add
rotated strings.

Also updated the comment as it's guaranteed that SimpleFold
returns values in increasing order.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/77730043

src/pkg/unicode/letter_test.go

@@ -387,32 +387,20 @@ func TestTurkishCase(t *testing.T) {
 }
 
 var simpleFoldTests = []string{
-	// SimpleFold could order its returned slices in any order it wants,
-	// but we know it orders them in increasing order starting at in
-	// and looping around from MaxRune to 0.
+	// SimpleFold(x) returns the next equivalent rune > x or wraps
+	// around to smaller values.
 
 	// Easy cases.
 	"Aa",
-	"aA",
 	"δΔ",
-	"Δδ",
 
 	// ASCII special cases.
 	"KkK",
-	"kKK",
-	"KKk",
 	"Ssſ",
-	"sſS",
-	"ſSs",
 
 	// Non-ASCII special cases.
 	"ρϱΡ",
-	"ϱΡρ",
-	"Ρρϱ",
 	"ͅΙιι",
-	"Ιιιͅ",
-	"ιιͅΙ",
-	"ιͅΙι",
 
 	// Extra special cases: has lower/upper but no case fold.
 	"İ",