Add Unit Tests for GC and strings (#209)

***NO_CI***

Tests/NFUnitTestGC/NFUnitTestGC.nfproj

@@ -25,6 +25,7 @@
   <Import Project="$(NanoFrameworkProjectSystemPath)NFProjectSystem.props" Condition="Exists('$(NanoFrameworkProjectSystemPath)NFProjectSystem.props')" />
   <ItemGroup>
     <Compile Include="TestGCWithByteArrays.cs" />
+    <Compile Include="TestGCWithStringArrays.cs" />
     <Compile Include="TestGCWithTimeSpanArrays.cs" />
     <Compile Include="TestGCWithDateTimeArrays.cs" />
     <Compile Include="TestGCWithObjectArrays.cs" />

Tests/NFUnitTestGC/TestGCWithStringArrays.cs

@@ -0,0 +1,103 @@
+//
+// Copyright (c) .NET Foundation and Contributors
+// Portions Copyright (c) Microsoft Corporation.  All rights reserved.
+// See LICENSE file in the project root for full license information.
+//
+
+using nanoFramework.TestFramework;
+using System;
+
+namespace NFUnitTestGC
+{
+    [TestClass]
+    public class TestGCWithStringArrays
+    {
+        [TestMethod]
+        public void TestCompactionForNotFixedStringArray()
+        {
+            OutputHelper.WriteLine("Starting TestCompactionForNotFixedStringArray");
+
+            for (int loop = 0; loop < 10; loop++)
+            {
+                OutputHelper.WriteLine($"Starting iteration {loop}");
+
+                // First we create objects and holes that keeps some space that could be used by compaction.
+
+                // Small count so compaction does not happen. 
+                HolderForString[] arrayOfStrings = new HolderForString[10];
+                RunStringAllocations(arrayOfStrings);
+
+                // This is the array that we expect to move in during compaction.
+                HolderForString[] testNativeBuffer = new HolderForString[100];
+                // Fill it, so it is not optimized out
+                for (int i = 0; i < testNativeBuffer.Length; i++)
+                {
+                    testNativeBuffer[i] = new HolderForString(Guid.NewGuid().ToString());
+                }
+
+                OutputHelper.WriteLine("Large HolderForString array created");
+                OutputHelper.WriteLine("Forcing compaction to occurr");
+
+                // Causes compaction
+                InitiateStringCompaction();
+
+                OutputHelper.WriteLine("Compaction occurred");
+                OutputHelper.WriteLine("Checking arrays for corrupted data...");
+
+                int index = 0;
+
+                // Check that array content is not corrupted
+                foreach (HolderForString holder in testNativeBuffer)
+                {
+                    Assert.AreEqual(
+                        holder.StringHash,
+                        holder.StringContent.GetHashCode(),
+                        $"Array content comparison failed at position {index}. Expecting {holder.StringHash}, found {holder.StringContent.GetHashCode()}");
+
+                    index++;
+                }
+
+                OutputHelper.WriteLine("No corruption detected in array");
+            }
+
+            OutputHelper.WriteLine("Completed TestCompactionForNotFixedArray");
+        }
+
+        // This function cause compaction to occur.
+        // It is not so trivial as it need to fragment heap with referenced objects.      
+        void InitiateStringCompaction()
+        {
+            // Large count, so compaction happens during RunAllocations.
+            HolderForString[] arrayOfArrays = new HolderForString[400];
+            RunStringAllocations(arrayOfArrays);
+        }
+
+        private void RunStringAllocations(HolderForString[] arrObj)
+        {
+            for (int i = 1; i < arrObj.Length; i++)
+            {
+                // Creates referenced object, which stays in memory until InitiateCompaction exits
+                arrObj[i] = new HolderForString(Guid.NewGuid().ToString());
+
+                // Tries to create larger object that would be later hole
+                // This object could be garbage collected on each "i" cycle
+                HolderForString[] arr = new HolderForString[50 * i];
+
+                // Creates some usage for array elements, so it is not optimized out
+                arr[0] = new HolderForString(Guid.NewGuid().ToString());
+            }
+        }
+
+        private class HolderForString
+        {
+            public int StringHash { get; }
+            public string StringContent { get; }
+
+            public HolderForString(string value)
+            {
+                StringContent = value;
+                StringHash = value.GetHashCode();
+            }
+        }
+    }
+}