Eclipse 2020.12 cannot run spring-boot-maven-plugin test

[philwebb]

Looks like we're hitting eclipse-buildship/buildship#980 (comment)

We should review the use of testRuntimeOnly("org.junit.platform:junit-platform-launcher")

[wilkinsona]

It'd be nice to do this via our conventions where we configure the use of JUnit Platform rather than having the dependency in every build script.