Fix failing regression test on x86-32 machines

Fix failing regression test on x86-32 machines

95d6e9af0 added code to display the tuplestore storage type for
WindowAgg nodes and added a test to ensure the "Disk" storage method was
working correctly by setting work_mem to 64 and running a test which
caused the WindowAgg to go to disk.  Seemingly, the number of rows
chosen there wasn't quite enough for that to happen in x86 32-bit.

Fix this by increasing the number of rows slightly.

I suspect the buildfarm didn't catch this as MEMORY_CONTEXT_CHECKING
builds will use a bit more memory for MemoryChunks to store the
requested_size and also because of the additional space to store the
chunk's sentinel byte.

Reported-by: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/Z-q3ZAM4OhE-4UiI@msg.df7cb.de