ttools: adjust taplint upload timestamp checking

Taplint now checks equivalence of parsed adql:TIMESTAMP values
between uploaded and retrieved tables, rather than of their
string serialization, so that services ingesting these timestamps
and reformatting them don't generate an error.

Also check that the format of returned timestamp columns matches
the TAP intention.

Also change the uploaded timestamp and floating point values to
test the data ingestion/selection for timestamp and floating point
values a little bit harder than before.

I could add timestamp format test elsewhere (query stages) so that
services not offering uploads also get this test; also for timestamp
querying as per TAP sec 2.3.4.  Haven't done it so far though.