Refactor and improve tests of pg_walinspect

The regression tests of pg_walinspect are reworked on a few aspects:
- Reorganization on the validation checks done for the start and end
LSNs on the six SQL functions currently available in 1.1.
- Addition of a few patterns doing bound checks for invalid start LSN,
invalid end LSN, and failures in reading LSN positions, for anything
that's been missing.
- Use of a consistent style across the whole, limiting blank lines
across the queries.
- Addition of a new test script for upgrades.  For the time being, this
is straight-forward with a check that the upgrade from 1.0 works
correctly.  This will be made more complicated once the interface of
this extension is reworked in 1.1 with a follow-up patch.

Most of the contents of this commit are extracted from a larger patch by
the same author, largely reorganized by me to minimize the differences
with the upcoming work aimed to lift the bound checks on the input LSNs
used by the functions of this extension.

Author: Bharath Rupireddy, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACU0_q-o4DSweyaW9NO1KBx-QkN6G_OzYQvpjf3CZVASkg@mail.gmail.com

contrib/pg_walinspect/sql/oldextversions.sql

@@ -0,0 +1,14 @@
+-- test old extension version entry points
+
+CREATE EXTENSION pg_walinspect WITH VERSION '1.0';
+
+-- List what version 1.0 contains
+\dx+ pg_walinspect
+
+-- Move to new version 1.1
+ALTER EXTENSION pg_walinspect UPDATE TO '1.1';
+
+-- List what version 1.1 contains
+\dx+ pg_walinspect
+
+DROP EXTENSION pg_walinspect;