injection_points: Add tests for name limits

injection_points: Add tests for name limits

The maximum limits for point name, library name, function name and
private area size were not kept track of in the tests.  The new function
introduced in 16a2f706951e gives a way to trigger them.  This is not
critical but cheap to cover.

While on it, this commit cleans up some of the tests introduced by
16a2f706951e for NULL inputs by using more consistent argument values.
The coverage does not change, but it makes the whole less confusing with
argument values that are correct based their position in the SQL
function called.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/aRE7zhu6wOA29gFf@paquier.xyz