bug#45539: 26.3; `add-to-ordered-list': Add optional arg for :test predicate for hash table

[Drew Adams]

Subject line says it all.

I had never heard of `add-to-ordered-list' before coming across this
user question:

https://emacs.stackexchange.com/q/62520/105

Why hard-code the hash-table :test predicate?  The fact of using a hash
table should be only an internal, implementation thing.  But the
comparison behavior for the ordering is a user thing.

Please consider adding an optional arg TEST-PREDICATE, which will be
passed to the hash table (and which will default to `eq').

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.18362
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''