Upgrade spring boot starter test to JUnit 5

[philwebb]

We're getting a lot of users asking for a dedicated JUnit 5 starter POM, but I'd rather we investigate how we can upgrade our existing starter to JUnit 5.

We need to consider the following use-cases:

• For green field applications it would be nice to use only JUnit 5. It will be confusing if there are multiple @Test imports.

• For existing applications it would be nice if they can migrate to JUnit 5 in their own time. An upgraded application should continue to work. It's likely we'll need to support this type of hybrid application for some time.

• It's possible that some users might want to remain on JUnit 4 only. We'll need to consider if this is something we want to support, or if it's something they should do themselves.

[philwebb]

See also #6402 (which this one might end up being a duplicate of)

[wilkinsona]

Following advice from the Gradle team, we've raised the minimum supported version of Gradle to 4.4 in Boot 2.1. Support for JUnit 5 was added to Gradle in 4.6 so moving to JUnit 5 by default will have a knock-on effect of requiring us to raise the minimum supported version of Gradle. I suspect that'll be fine post 2.1.

[wilkinsona]

Eclipse does not yet fully support JUnit 5 out of the box. To use it with 2018-09 you either have to install a patch or also have JUnit 4 on the classpath. See https://bugs.eclipse.org/bugs/show_bug.cgi?id=538956 for details. The problem should be fixed in 2018-12.

[sbrannen]

Support for JUnit 5 was added to Gradle in 4.6 so moving to JUnit 5 by default will have a knock-on effect of requiring us to raise the minimum supported version of Gradle.

Please note as well that there have been bug fixes in Gradle's support for JUnit 5 in subsequent Gradle releases as well. So, Gradle 4.6 would be the bare minimum.