Add base shell test system

- NOTE: very much wip and unstable
- This commit is a first step to provide boot style
  @ShellTest annotation
- New modules spring-shell-test and spring-shell-test-autoconfigure
- Focus is to autoconfigure context without shell runners so that
  we can create "sessions" and hook to configures jline terminal
  with custom in/out streams.
- Skeleton fork from jediterm to provide basic terminal emulation
  to part of a control amd escape characters working.
- ShellTestClient is a concept user can use to interact with a shell
  in a same way user would use a "real" shell.
- Fixes #489

spring-shell-docs/spring-shell-docs.gradle

@@ -7,7 +7,9 @@ description = 'Spring Shell Documentation'
 dependencies {
 	management platform(project(":spring-shell-management"))
 	implementation project(':spring-shell-starters:spring-shell-starter')
+	implementation project(':spring-shell-starters:spring-shell-starter-test')
 	testImplementation 'org.springframework.boot:spring-boot-starter-test'
+	testImplementation 'org.awaitility:awaitility'
 }
 
 asciidoctorj {