Document support for merging TELs with defaults

This commit restructures the content of the "TestExecutionListener
registration and ordering" section of the Testing chapter in the
reference manual into cross-referenced subsections with more
appropriate titles.

In addition, this commit introduces a new "Merging
TestExecutionListeners" subsection which documents the new MergeMode
feature in @TestExecutionListeners, including an example demonstrating
the MERGE_WITH_DEFAULTS mode.

Issue: SPR-12083