[#283] pkg/session: Cover Token's Sign/Verify methods with unit test

Add `sessiontest.GenerateSigned` function which returns signed random token.
Clarify that `sessiontest.Generate` returns an unsigned token. Use these
functions to assert the correctness of `Sign` / `VerifySignature` methods.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/session/test/token.go

@@ -11,6 +11,8 @@ import (
 )
 
 // Generate returns random session.Token.
+//
+// Resulting token is unsigned.
 func Generate() *session.Token {
 	tok := session.NewToken()
 
@@ -33,3 +35,17 @@ func Generate() *session.Token {
 
 	return tok
 }
+
+// GenerateSigned returns signed random session.Token.
+//
+// Panics if token could not be signed (actually unexpected).
+func GenerateSigned() *session.Token {
+	tok := Generate()
+
+	err := tok.Sign(test.DecodeKey(0))
+	if err != nil {
+		panic(err)
+	}
+
+	return tok
+}