Unify method visibility of private classes

Apply checkstyle rule to ensure that private and package private
classes do not have unnecessary public methods. Test classes have
also been unified as much as possible to use default scoped
inner-classes.

Closes gh-7316

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/PropertiesLauncherTests.java

@@ -360,7 +360,7 @@ class PropertiesLauncherTests {
 		};
 	}
 
-	public static class TestLoader extends URLClassLoader {
+	static class TestLoader extends URLClassLoader {
 
 		TestLoader(ClassLoader parent) {
 			super(new URL[0], parent);

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jar/CentralDirectoryParserTests.java

@@ -88,7 +88,7 @@ class CentralDirectoryParserTests {
 		assertThat(headers.hasNext()).isFalse();
 	}
 
-	private static class Collector implements CentralDirectoryVisitor {
+	static class Collector implements CentralDirectoryVisitor {
 
 		private List<CentralDirectoryFileHeader> headers = new ArrayList<>();
 
@@ -105,13 +105,13 @@ class CentralDirectoryParserTests {
 		public void visitEnd() {
 		}
 
-		public List<CentralDirectoryFileHeader> getHeaders() {
+		List<CentralDirectoryFileHeader> getHeaders() {
 			return this.headers;
 		}
 
 	}
 
-	private static class MockCentralDirectoryVisitor implements CentralDirectoryVisitor {
+	static class MockCentralDirectoryVisitor implements CentralDirectoryVisitor {
 
 		private final List<String> invocations = new ArrayList<>();
 
@@ -130,7 +130,7 @@ class CentralDirectoryParserTests {
 			this.invocations.add("visitEnd");
 		}
 
-		public List<String> getInvocations() {
+		List<String> getInvocations() {
 			return this.invocations;
 		}