Aggregate JUnit and TestNG test reports

Prior to this commit, the test results for JUnit tests overwrote the
results for TestNG tests in the HTML test report generated by the
Gradle build.

This commit addresses this problem by introducing a new
'aggregateTestReports' task in the spring-test module and making the
'check' task depend on it.

To see aggregated reports in the spring-test module, execute either
`gradle build` or `gradle check`. Executing `gradle test` alone will
not trigger test report aggregation.

Issue: SPR-11509