Rework new SLRU test with injection points

Rather than the SQL injection_points_load(), this commit changes the
injection point test introduced in 768a9fd5535f to rely on the two
macros INJECTION_POINT_LOAD() and INJECTION_POINT_CACHED(), that have
been originally introduced for the sake of this test.

This runs the test as a two-step process: load the injection point, then
run its callback directly from the local cache loaded.  What the test
did originally was also fine, but the point here is to have an example
in core of how to use these new macros.

While on it, fix the header ordering in multixact.c, as pointed out by
Alexander Korotkov.  This was an oversight in 768a9fd5535f.

Per discussion with Álvaro Herrera.

Author: Michael Paquier
Discussion: https://postgr.es/m/ZsUnJUlSOBNAzwW1@paquier.xyz
Discussion: https://postgr.es/m/CAPpHfduzaBz7KMhwuVOZMTpG=JniPG4aUosXPZCxZydmzq_oEQ@mail.gmail.com

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