test: types: add test cases for timestamp_type to_string format
test: types: add test cases for timestamp_type to_string format

Following the previous patch that changed time_point_to_string
we should cement the different edge cases for the next
time this function changes.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla