In this PR, we fix two bugs present in `boost_test_tree_lister` that
affected the output of `--list_json_content` added in
scylladb/scylladb@afde5f668a:
* The labels test units use were duplicated in the output.
* If a test suite or a test file didn't contain any tests, it wasn't
listed in the output.
Refs scylladb/scylladb#25415
Backport: not needed. The code hasn't been used anywhere yet.
Closesscylladb/scylladb#28255
* github.com:scylladb/scylladb:
test/lib/boost_test_tree_lister.cc: Record empty test suites
test/lib/boost_test_tree_lister.cc: Deduplicate labels