Improve DeltaDataTreeTest and migrate to JUnit 5 eclipse-platform#903

This migrates the DeltaDataTreeTest to JUnit 5 and improves the test
cases implementation.

The test case implementations are improved as follows:
- Replace manual exception processing with assertThrows() or having the
method throw the exception to be handles by the JUnit framework
- Replace assertTrue()/assertFalse() with assertThat().matches() to
improve error messages

Contributes to
eclipse-platform#903

resources/tests/org.eclipse.core.tests.resources/META-INF/MANIFEST.MF

@@ -37,6 +37,7 @@ Require-Bundle: org.eclipse.core.resources,
 Import-Package: org.assertj.core.api,
  org.junit.jupiter.api,
  org.junit.jupiter.api.extension,
+ org.junit.jupiter.api.function,
  org.junit.jupiter.api.io,
  org.junit.platform.suite.api,
  org.mockito