test: pylib: fix `read_barrier` implementation

The previous implementation didn't actually do a read barrier, because
the statement failed on an early prepare/validate step which happened
before read barrier was even performed.

Change it to a statement which does not fail and doesn't perform any
schema change but requires a read barrier.

This breaks one test which uses `RandomTables.verify_schema()` when only
one node is alive, but `verify_schema` performs a read barrier. Unbreak
it by skipping the read barrier in this case (it makes sense in this
particular test).

Closes #13933