Add Speedbar tests (bug#73533)

Add Speedbar tests (bug#73533)

Add Speedbar tests which test the operation of
'speedbar-expand-line-descendants'.
* test/lisp/speedbar-tests.el (speedbar-tests-container)
(eieio-speedbar-object-children, speedbar-tests-item)
(speedbar-tests--make-object, speedbar-tests--setup-strings)
(speedbar-tests--object-hierarchy, speedbar-tests--base-items)
(speedbar-tests--clean-up, speedbar-tests--initialize)
(speedbar-tests--object-name-expanded)
(speedbar-tests--object-name-function)
(speedbar-tests--objects-as-strings)
(speedbar-tests--state-test)
(speedbar-tests--expand-descendants-single)
(speedbar-tests--expand-descendants-nested)
(speedbar-tests--expand-descendants-nested-wide)
(speedbar-tests--expand-descendants-of-first)
(speedbar-tests--expand-descendants-of-first-expanded)
(speedbar-tests--expand-descendants-of-last)
(speedbar-tests--expand-descendants-of-last-expanded)
(speedbar-tests--expand-descendants-of-middle)
(speedbar-tests--expand-descendants-of-middle-expanded):
New tests, test 'speedbar-expand-line-descendants'.

(cherry picked from commit 9dcc32f10cbae9497d6b33fcc739b75c1d5e411c)