Fix crash when joining a dedicated server

We can't use FriendlyByte.readCollection to read to a
pre-allocated/array-backed NonNullList, as that doesn't implement
List.add. Instead, we just need to do a normal loop.

We add a couple of tests to round-trip our recipe specs. Unfortunately
we can't test the recipes themselves as our own registries aren't set
up, so this'll have to do for now.

projects/forge/build.gradle.kts

@@ -165,6 +165,8 @@ dependencies {
     testModImplementation(testFixtures(project(":core")))
     testModImplementation(testFixtures(project(":forge")))
 
+    testFixturesImplementation(testFixtures(project(":core")))
+
     "cctJavadoc"(libs.cctJavadoc)
 }