Add option to specify segment size in blocks

Add option to specify segment size in blocks

The tests don't have much coverage of segment related code, as we don't create
large enough tables. To make it easier to test these paths, add a new option
specifying the segment size in blocks.

Set the new option to 6 blocks in one of the CI tasks. Smaller numbers
currently fail one of the tests, for understandable reasons.

While at it, fix some segment size related issues in the meson build.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20221107171355.c23fzwanfzq2pmgt@awork3.anarazel.de