tests/types_test: Fix int32 type string conversion boundary case
tests/types_test: Fix int32 type string conversion boundary case

The test case is interested in the upper boundary of 32-bit integer
because we already test the lower boundary in assertions below. The old
test passed, of course, but it wasn't very interesting.
Message-Id: <7c41f36b10e5ff7ec115b44c7799ae1f1375c7fd@scylladb.com>

duarten/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,scylladb/scylla