Remove remaining assumption about File separator from LogFileTests

See gh-15471

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/LogFileTests.java

@@ -93,7 +93,7 @@ public class LogFileTests {
 		assertThat(logFile.toString())
 				.isEqualTo("logpath" + File.separatorChar + "spring.log");
 		assertThat(properties.getProperty(LoggingSystemProperties.LOG_FILE))
-				.isEqualTo("logpath/spring.log");
+				.isEqualTo("logpath" + File.separatorChar + "spring.log");
 		assertThat(properties.getProperty(LoggingSystemProperties.LOG_PATH))
 				.isEqualTo("logpath");
 	}