Allow game to specify first and last mod in mod loading order (#14177)

Co-authored-by: Lars Mueller <appgurulars@gmx.de>
Co-authored-by: sfan5 <sfan5@live.de>

games/devtest/game.conf

@@ -1,2 +1,4 @@
 title = Development Test
 description = Testing environment to help with testing the engine features of Minetest. It can also be helpful in mod development.
+first_mod = first_mod
+last_mod = last_mod

games/devtest/mods/first_mod/init.lua

@@ -0,0 +1 @@
+-- Nothing to do here, loading order is tested in C++ unittests.

games/devtest/mods/first_mod/mod.conf

@@ -0,0 +1,2 @@
+name = first_mod
+description = Mod which should be loaded before every other mod.

games/devtest/mods/last_mod/init.lua

@@ -0,0 +1 @@
+-- Nothing to do here, loading order is tested in C++ unittests.

games/devtest/mods/last_mod/mod.conf

@@ -0,0 +1,5 @@
+name = last_mod
+description = Mod which should be loaded as last mod.
+# Test dependencies
+optional_depends = unittests
+depends = first_mod

games/devtest/mods/unittests/mod.conf

@@ -1,3 +1,4 @@
 name = unittests
 description = Adds automated unit tests for the engine
-depends = basenodes
+# Also test that it is possible to depend on first_mod
+depends = first_mod, basenodes