Fix fluid testmod, add dummy init method to FluidStack (classloading 😍)

Signed-off-by: Max <maxh2709@gmail.com>

common/src/main/java/dev/architectury/fluid/FluidStack.java

@@ -244,4 +244,8 @@ public final class FluidStack {
         if (isEmpty()) return this;
         return new FluidStack(getRawFluidSupplier(), amount, getTag());
     }
+    
+    public static void init() {
+        // classloading my beloved 😍
+    }
 }