make UseSharedConnection available in TesstConfig.json; defaults to true

tests/StackExchange.Redis.Tests/BasicOps.cs

@@ -402,6 +402,7 @@ private void Incr(IDatabase database, RedisKey key, int delta, ref int total)
         [Fact]
         public void ShouldUseSharedMuxer()
         {
+            Writer.WriteLine($"Shared: {SharedFixtureAvailable}");
             if (SharedFixtureAvailable)
             {
                 using (var a = Create())

tests/StackExchange.Redis.Tests/Helpers/TestConfig.cs

@@ -43,6 +43,7 @@ static TestConfig()
 
         public class Config
         {
+            public bool UseSharedConnection { get; set; } = true;
             public bool RunLongRunning { get; set; }
             public bool LogToConsole { get; set; }
 

tests/StackExchange.Redis.Tests/SharedConnectionFixture.cs

@@ -12,7 +12,7 @@ namespace StackExchange.Redis.Tests
 {
     public class SharedConnectionFixture : IDisposable
     {
-        public bool IsEnabled => true; // set to false to disable globally
+        public bool IsEnabled { get; }
 
         public const string Key = "Shared Muxer";
         private readonly ConnectionMultiplexer _actualConnection;
@@ -21,6 +21,7 @@ public class SharedConnectionFixture : IDisposable
 
         public SharedConnectionFixture()
         {
+            IsEnabled = TestConfig.Current.UseSharedConnection;
             Configuration = TestBase.GetDefaultConfiguration();
             _actualConnection = TestBase.CreateDefault(
                 output: null,