Use String.repeat() where feasible

See gh-31916

spring-messaging/src/test/java/org/springframework/messaging/simp/stomp/StompHeaderAccessorTests.java

@@ -239,13 +239,10 @@ public class StompHeaderAccessorTests {
 		String actual = accessor.getShortLogMessage("payload".getBytes(StandardCharsets.UTF_8));
 		assertThat(actual).isEqualTo("SEND /foo session=123 application/json payload=payload");
 
-		StringBuilder sb = new StringBuilder();
-		for (int i = 0; i < 80; i++) {
-			sb.append('a');
-		}
-		final String payload = sb.toString() + " > 80";
+		String string = "a".repeat(80);
+		final String payload = string + " > 80";
 		actual = accessor.getShortLogMessage(payload.getBytes(StandardCharsets.UTF_8));
-		assertThat(actual).isEqualTo(("SEND /foo session=123 application/json payload=" + sb + "...(truncated)"));
+		assertThat(actual).isEqualTo(("SEND /foo session=123 application/json payload=" + string + "...(truncated)"));
 	}
 
 }

spring-web/src/test/java/org/springframework/http/codec/protobuf/ProtobufDecoderTests.java

@@ -162,11 +162,7 @@ public class ProtobufDecoderTests extends AbstractDecoderTests<ProtobufDecoder>
 	@SuppressWarnings("deprecation")
 	public void decodeSplitMessageSize() {
 		this.decoder.setMaxMessageSize(100009);
-		StringBuilder builder = new StringBuilder();
-		for (int i = 0; i < 10000; i++) {
-			builder.append("azertyuiop");
-		}
-		Msg bigMessage = Msg.newBuilder().setFoo(builder.toString()).setBlah(secondMsg2).build();
+		Msg bigMessage = Msg.newBuilder().setFoo("azertyuiop".repeat(10000)).setBlah(secondMsg2).build();
 
 		Flux<DataBuffer> input = Flux.just(bigMessage, bigMessage)
 				.flatMap(msg -> Mono.defer(() -> {