Fix file handle leak in JarFileTests

The JarFile was not being closed which linked a file handle and caused
a test failure on Windows.

The local variable has been renamed as, when declared in a
try-with-resources, Checkstyle was confused by the shadowing of the
jarFile field and required references to jarFile within the try-block
the be prefixed with this.

See gh-19595

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

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -511,11 +511,12 @@ public class JarFileTests {
 			jarOutputStream.write(new byte[] { (byte) 1 });
 			jarOutputStream.closeEntry();
 		}
-		JarFile jarFile = new JarFile(file);
-		Enumeration<java.util.jar.JarEntry> entries = jarFile.entries();
-		JarEntry entry = entries.nextElement();
-		assertThat(entry.getLastModifiedTime().toInstant()).isEqualTo(Instant.EPOCH);
-		assertThat(entry.getName()).isEqualTo("1.dat");
+		try (JarFile jar = new JarFile(file)) {
+			Enumeration<java.util.jar.JarEntry> entries = jar.entries();
+			JarEntry entry = entries.nextElement();
+			assertThat(entry.getLastModifiedTime().toInstant()).isEqualTo(Instant.EPOCH);
+			assertThat(entry.getName()).isEqualTo("1.dat");
+		}
 	}
 
 	private int getJavaVersion() {