Move some tests ouf of sample app into tests

Samples/SwiftJavaExtractFFMSampleApp/Sources/MySwiftLibrary/MySwiftLibrary.swift

@@ -63,6 +63,24 @@ public func withBuffer(body: (UnsafeRawBufferPointer) -> Void) {
   body(globalBuffer)
 }
 
+public func getArray() -> [UInt8] {
+  return [1, 2, 3]
+}
+
+public func sumAllByteArrayElements(actuallyAnArray: UnsafeRawPointer, count: Int) -> Int {
+  let bufferPointer = UnsafeRawBufferPointer(start: actuallyAnArray, count: count)
+  let array = Array(bufferPointer)
+  return Int(array.reduce(0, { partialResult, element in partialResult + element }))
+}
+
+public func sumAllByteArrayElements(array: [UInt8]) -> Int {
+  return Int(array.reduce(0, { partialResult, element in partialResult + element }))
+}
+
+public func withArray(body: ([UInt8]) -> Void) {
+  body([1, 2, 3])
+}
+
 public func globalReceiveSomeDataProtocol(data: some DataProtocol) -> Int {
   p(Array(data).description)
   return data.count

Samples/SwiftJavaExtractFFMSampleApp/src/main/java/com/example/swift/HelloJava2Swift.java

@@ -52,9 +52,7 @@ static void examples() {
 
         CallTraces.trace("getGlobalBuffer().byteSize()=" + MySwiftLibrary.getGlobalBuffer().byteSize());
 
-        MySwiftLibrary.withBuffer((buf) -> {
-            CallTraces.trace("withBuffer{$0.byteSize()}=" + buf.byteSize());
-        });
+
         // Example of using an arena; MyClass.deinit is run at end of scope
         try (var arena = AllocatingSwiftArena.ofConfined()) {
             MySwiftClass obj = MySwiftClass.init(2222, 7777, arena);

Samples/SwiftJavaExtractFFMSampleApp/src/test/java/com/example/swift/WithBufferTest.java

@@ -0,0 +1,60 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+package com.example.swift;
+
+import org.junit.jupiter.api.Test;
+import org.swift.swiftkit.core.*;
+import org.swift.swiftkit.ffm.*;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.lang.foreign.ValueLayout;
+import java.util.Arrays;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.stream.IntStream;
+
+public class WithBufferTest {
+    @Test
+    void test_withBuffer() {
+        AtomicLong bufferSize = new AtomicLong();
+        MySwiftLibrary.withBuffer((buf) -> {
+            CallTraces.trace("withBuffer{$0.byteSize()}=" + buf.byteSize());
+            bufferSize.set(buf.byteSize());
+        });
+
+        assertEquals(124, bufferSize.get());
+    }
+
+    @Test
+    void test_sumAllByteArrayElements_throughMemorySegment() {
+        byte[] bytes =  new byte[124];
+        Arrays.fill(bytes, (byte) 1);
+
+        try (var arena = AllocatingSwiftArena.ofConfined()) {
+            // NOTE: We cannot use MemorySegment.ofArray because that creates a HEAP backed segment and therefore cannot pass into native:
+            //         java.lang.IllegalArgumentException: Heap segment not allowed: MemorySegment{ kind: heap, heapBase: [B@5b6ec132, address: 0x0, byteSize: 124 }
+            //            MemorySegment bytesSegment = MemorySegment.ofArray(bytes); // NO COPY (!)
+            //            MySwiftLibrary.sumAllByteArrayElements(bytesSegment, bytes.length);
+
+            var bytesCopy = arena.allocateFrom(ValueLayout.JAVA_BYTE, bytes);
+            var swiftSideSum = MySwiftLibrary.sumAllByteArrayElements(bytesCopy, bytes.length);
+
+            System.out.println("swiftSideSum = " + swiftSideSum);
+
+            int javaSideSum = IntStream.range(0, bytes.length).map(i -> bytes[i]).sum();
+            assertEquals(javaSideSum, swiftSideSum);
+        }
+    }
+}