Fix AssemblyLoadContext fixture

Resolves #676.

test/csharp/fixture_AssemblyLoadContext.cs

@@ -0,0 +1,26 @@
+using Xunit;
+using System;
+using System.Management.Automation;
+using Microsoft.PowerShell.Linux.Host;
+
+// This collection fixture initializes Core PowerShell's AssemblyLoadContext once and only
+// once. Attempting to initialize in a class level fixture will cause multiple
+// initializations, resulting in test failure due to "Binding model is already locked for
+// the AppDomain and cannot be reset".
+namespace PSTests
+{
+    public class AssemblyLoadContextFixture
+    {
+        public AssemblyLoadContextFixture()
+        {
+            // Initialize the Core PowerShell AssemblyLoadContext
+            PowerShellAssemblyLoadContextInitializer.SetPowerShellAssemblyLoadContext(AppContext.BaseDirectory);
+        }
+    }
+
+    [CollectionDefinition("AssemblyLoadContext")]
+    public class AssemblyLoadContextCollection : ICollectionFixture<AssemblyLoadContextFixture>
+    {
+        // nothing to do but satisfy the interface
+    }
+}

test/csharp/test_Runspace.cs

@@ -9,17 +9,12 @@ namespace PSTests
     // NOTE: do not call AddCommand("out-host") after invoking or MergeMyResults,
     // otherwise Invoke will not return any objects
 
+    [Collection("AssemblyLoadContext")]
     public class RunspaceTests
     {
         private static int count = 3;
         private static string script = String.Format($"get-process | select-object -first {count}");
 
-        // Initialize the Core PowerShell AssemblyLoadContext
-        public RunspaceTests()
-        {
-            PowerShellAssemblyLoadContextInitializer.SetPowerShellAssemblyLoadContext(AppContext.BaseDirectory);
-        }
-
         [Fact]
         public void TestRunspaceWithPipeline()
         {