Remove unstable testing features #1225
Description
As of 3.4.1, there are some unstable shell testing features that are shipped in a stable release, eg 3.4.1: https://docs.spring.io/spring-shell/reference/3.4/testing/index.html
Testing support is currently under development and will be unstable for various parts.
Unstable features should not be shipped in a stable release. The upcoming v4.0 should contain only stable features. Unstable ones should removed, then stabilized and released in later minor releases 4.1, 4.2, etc