When run on a server using default_toast_compression set to LZ4, this
test would fail because of a consistency issue with the order of the
tuples treated.  LZ4 causes one tuple to be stored inline instead of
getting externalized.  As the goal of this test is to check after data
stored externally, stick to pglz as the compression algorithm used, so
as all data of this test is stored the way it should.

Analyzed-by: Dilip Kumar
Discussion: https://postgr.es/m/YLrDWxJgM8WWMoCg@paquier.xyz