SpringBoot Polish access modifiers for test classes

This PR polishes access modifiers for test classes.

Comment From: philwebb

I thought we has a checkstyle for this at one point. Perhaps it's not working or I'm misremembering.

Comment From: dreis2211

@philwebb I think this just checks for method visibility. See https://github.com/spring-io/spring-javaformat/blob/main/spring-javaformat/spring-javaformat-checkstyle/src/main/java/io/spring/javaformat/checkstyle/check/SpringJUnit5Check.java#L153

Comment From: philwebb

Thanks @dreis2211!

Comment From: wilkinsona

Thanks very much, @izeye.