Add tests and more context in the StringRules file

error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/StringRules.java

@@ -246,5 +246,9 @@ final class StringRules {
   }
 
   // Rewrite `String#copyValueOf(char[])` to `new String(char[])`.
+  // Example:
+  // String.copyValueOf(new char[]);
+  // -->
+  // new String(new char[]);
   
 }

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/StringRulesTestInput.java

@@ -96,4 +96,8 @@ final class StringRulesTest implements RefasterRuleCollectionTestCase {
   int testUtf8EncodedLength() {
     return "foo".getBytes(UTF_8).length;
   }
+
+  String testNewStringChar() {
+    return String.copyValueOf(new char[] {'f', 'o', 'o'});
+  }
 }

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/StringRulesTestOutput.java

@@ -96,4 +96,8 @@ final class StringRulesTest implements RefasterRuleCollectionTestCase {
   int testUtf8EncodedLength() {
     return Utf8.encodedLength("foo");
   }
+
+  String testNewStringChar() {
+    return new String(new char[] {'f', 'o', 'o'});
+  }
 }