This commit eliminates the 'integration-tests' subproject in favor of
managing these sources under the root project's own 'src' directory.
This helps to avoid special-case handling for integration-tests in the
Gradle build, e.g. avoiding publication of jars to Artifactory /
Maven Central.
It is also semantically more correct. This is not a Spring Framework
subproject so much as it is a collection of integration tests that
span functionality across many subprojects. In this way, it makes
sense to place them directly under the root project.
Issue: SPR-8116
