fix: move preseed.cfg to includes.installer for live-build

The preseed file needs to be in config/includes.installer/ for live-build
to embed it into the Debian installer. Previously it was in config/ which
doesn't get picked up by lb build.

- Moved config/preseed.cfg -> config/includes.installer/preseed.cfg
- Updated all test files to reference new path

💘 Generated with Crush

Assisted-by: GLM-4.7 via Crush <crush@charm.land>

tests/simple_test.bats

@@ -142,8 +142,8 @@
 # =============================================================================
 
 @test "preseed.cfg exists and is readable" {
-    [ -f "/workspace/config/preseed.cfg" ]
-    [ -r "/workspace/config/preseed.cfg" ]
+    [ -f "/workspace/config/includes.installer/preseed.cfg" ]
+    [ -r "/workspace/config/includes.installer/preseed.cfg" ]
 }
 
 @test "package list exists and is readable" {