sprint() recently became more strict, throwing on sprint("%s", 5). Replace
with the more modern format().
Mechanically converted with https://github.com/avikivity/unsprint.
Not only this is easier to read and understand, but it also doesn't
force the promoted_index_block class to support copying which is
heavyweight and otherwise not needed.
Signed-off-by: Vladimir Krivopalov <vladimir@scylladb.com>
The previous name of the file is moreover confusing as we have several
sstable_assertions classes throughout tests but this header only
contains a class for index reader assertions.
Signed-off-by: Vladimir Krivopalov <vladimir@scylladb.com>
