Remove restriction on SQL block length in isolationtester scanner.

Remove restriction on SQL block length in isolationtester scanner.

specscanner.l had a fixed limit of 1024 bytes on the length of
individual SQL stanzas in an isolation test script.  People are
starting to run into that, so fix it by making the buffer resizable.

Once we allow this in HEAD, it seems inevitable that somebody will
try to back-patch a test that exceeds the old limit, so back-patch
this change as a preventive measure.

Daniel Gustafsson

Discussion: https://postgr.es/m/8D628BE4-6606-4FF6-A3FF-8B2B0E9B43D0@yesql.se