The tables produced by `storage/flux` didn't previously pass our table
tests. The `Empty()` call is supposed to return false if the table was
ever not empty, but reading the table or calling `Done()` would cause
the table implementations here to return that they were always empty.
This messes up the csv encoder which then believes that it just emitted
an empty table.
The table tests for valid table implementations states that this is an
error for the table implementation. This change introduces a simple test
for `ReadFilter` and also runs the table tests on the filter iterator.
Jonathan A. Sternberg