Fix pg_basebackup test to original intent

One test case was meant to check that pg_basebackup does not succeed
when a slot is specified with -S but WAL streaming is not selected,
which used to require specifying -X stream.  Since -X stream is the
default in PostgreSQL 10, this test case no longer covers that meaning,
but the pg_basebackup invocation happened to fail anyway for the
unrelated reason that the specified replication slot does not exist.  To
fix, move the test case to later in the file where the slot does exist,
and add -X none to the invocation so that it covers the originally meant
behavior.

extracted from a patch by Michael Banck <michael.banck@credativ.de>