Merge pull request #850 from PowerShell/andschwa/fix-xunit

Re-enable xUnit tests on Linux

src/Microsoft.PowerShell.CoreCLR.AssemblyLoadContext/CoreCLR/CorePsAssemblyLoadContext.cs

@@ -522,7 +522,15 @@ namespace System.Management.Automation
             if (!IsInitialized)
             {
                 var psAsmLoadContext = new PowerShellAssemblyLoadContext(basePaths);
-                AssemblyLoadContext.InitializeDefaultContext(psAsmLoadContext);
+                try
+                {
+                    AssemblyLoadContext.InitializeDefaultContext(psAsmLoadContext);
+                }
+                catch (System.InvalidOperationException)
+                {
+                    // We may not be able to set the default context. If we're under the
+                    // xUnit test harness, it has already been set.
+                }
                 IsInitialized = true;
             }
         }