Apply SingleSpaceSeparator Checkstyle module

This commit also fixes its violations.

Closes gh-31469

spring-core/src/test/java/org/springframework/core/BridgeMethodResolverTests.java

@@ -252,7 +252,7 @@ class BridgeMethodResolverTests {
 		Method bridgedMethod = MegaMessageProducerImpl.class.getDeclaredMethod("receive", MegaMessageEvent.class);
 		assertThat(bridgedMethod.isBridge()).isFalse();
 
-		Method bridgeMethod  = MegaMessageProducerImpl.class.getDeclaredMethod("receive", MegaEvent.class);
+		Method bridgeMethod = MegaMessageProducerImpl.class.getDeclaredMethod("receive", MegaEvent.class);
 		assertThat(bridgeMethod.isBridge()).isTrue();
 
 		assertThat(BridgeMethodResolver.findBridgedMethod(bridgeMethod)).isEqualTo(bridgedMethod);

spring-core/src/test/java/org/springframework/core/codec/CharBufferDecoderTests.java

@@ -128,7 +128,7 @@ class CharBufferDecoderTests extends AbstractDecoderTests<CharBufferDecoder> {
 	}
 
 	@Test
-	void decodeNewlinesAcrossBuffers()  {
+	void decodeNewlinesAcrossBuffers() {
 		Flux<DataBuffer> input = Flux.just(
 				stringBuffer("\r"),
 				stringBuffer("\n"),

spring-core/src/test/java/org/springframework/core/codec/StringDecoderTests.java

@@ -139,7 +139,7 @@ class StringDecoderTests extends AbstractDecoderTests<StringDecoder> {
 	}
 
 	@Test
-	void decodeNewlinesAcrossBuffers()  {
+	void decodeNewlinesAcrossBuffers() {
 		Flux<DataBuffer> input = Flux.just(
 				stringBuffer("\r"),
 				stringBuffer("\n"),

spring-core/src/test/java/org/springframework/core/convert/support/GenericConversionServiceTests.java

@@ -633,7 +633,7 @@ class GenericConversionServiceTests {
 	}
 
 
-	private static class MyStringToIntegerArrayConverter implements Converter<String, Integer[]>	{
+	private static class MyStringToIntegerArrayConverter implements Converter<String, Integer[]> {
 
 		@Override
 		public Integer[] convert(String source) {

spring-core/src/test/java/org/springframework/core/io/support/PathMatchingResourcePatternResolverTests.java

@@ -200,7 +200,7 @@ class PathMatchingResourcePatternResolverTests {
 			}
 
 			@Test
-			void usingFileProtocolWithoutWildcardInPatternAndEndingInSlashStarStar()  {
+			void usingFileProtocolWithoutWildcardInPatternAndEndingInSlashStarStar() {
 				Path testResourcesDir = Paths.get("src/test/resources").toAbsolutePath();
 				String pattern = String.format("file:%s/scanned-resources/**", testResourcesDir);
 				String pathPrefix = ".+?resources/";