Ensure all tests in spring-test are executed

Prior to this commit TestNG tests would only be executed by the Gradle
build if they were located in the “testng” package. Tests in subpackages
would therefore be omitted from the build.

This commit ensures that all TestNG classes in the “testng” package and
any of its subpackages are executed in the Gradle build.

Furthermore, this commit ensures that the JUnit-based
FailingBeforeAndAfterMethodsTests test class is executed along with the
other JUnit tests even though it resides under the “testng” package.

Issue: SPR-11338