Add "const" attributes to `assignment_testable::test_assignment` and `term::raw::prepare` methods. These should have been marked as "const" even before the change but for some reason were missing these qualifiers. Mark other supplementary methods with "const" attributes as necessary. Tests: unit(dev, debug) Signed-off-by: Pavel Solodovnikov <pa.solodovnikov@scylladb.com> Message-Id: <20200127213215.494000-1-pa.solodovnikov@scylladb.com>