INT-3543: Remove `context.close()` from tests

JIRA: https://jira.spring.io/browse/INT-3543

Quoting Sam Brannen:

> However, one should *never* programmatically close the injected application context in a test,
since the Spring TestContext Framework caches all contexts across the entire JVM process.
If you need to close a context after a test method or test class for some reason,
the only supported and reliable mechanism is `@DirtiesContext`.

**Cherry-pick to 4.0.x**
S-G Doc Polishing
Merge branch 'master' of https://github.com/spring-projects/spring-integration into INT-3538
Merge branch 'INT-3538' of https://github.com/garyrussell/spring-integration into INT-3538
INT-3538 Add `requires-reply` to `scatter-gather`