tests: devicetree: remove confusing/useless strings

Many of the messages that are printed on failed test assertions in the
devicetre API tests provide little value (e.g. merely restating some
expression in the test) or are outright confusing (do they print the
thing that should have happened, or what went wrong?).

The ztest framework already prints the expression that failed, so
there's no need for this.

There are some exceptions, like in test_dep_ord(), where the messages
do print useful information, namely failing array indexes.

Keep those, but give the file a cleanup by removing the cruft. There
are alread cases where we use "" or NULL for self-explanatory
assertions in this file; we're just expanding that practice.

No changes to test results expected.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>