Fix race condition inside DataCollectionAttachmentManager

[MarcoRossignoli]

• We're analyzing a strange race condition issue, but if we enable diag logs we're hiding real issue for an NRE inside the logs self.

• Update Dictionary to ConcurrentDictionary to make DataCollectionAttachmentManager thread safe, concurrent call to the data collector TestCaseEnded will eventually fail.

 [TestMethod]
    public void ParallelAccess()
    {
        _attachmentManager.Initialize(new SessionId(Guid.NewGuid()), Path.GetTempPath(), _messageSink.Object);
        Task.WaitAll(

        Task.Run(() =>
        {
            try
            {
                while (true)
                {
                    var dc = new DataCollectionContext(new SessionId(Guid.NewGuid()));
                    string path = Path.GetTempFileName();
                    File.WriteAllText(path, "test");
                    _attachmentManager.AddAttachment(new FileTransferInformation(dc, path, true), null, new Uri("//test1"), "test1");
                    var res = _attachmentManager.GetAttachments(dc);
                }
            }
            catch (Exception ex)
            {
                Assert.Fail(ex.ToString());
            }
        }),

        Task.Run(() =>
        {
            try
            {
                while (true)
                {
                    var dc = new DataCollectionContext(new SessionId(Guid.NewGuid()));
                    string path = Path.GetTempFileName();
                    File.WriteAllText(path, "test");
                    _attachmentManager.AddAttachment(new FileTransferInformation(dc, path, true), null, new Uri("//test2"), "test2");
                    var res = _attachmentManager.GetAttachments(dc);
                }
            }
            catch (Exception ex)
            {
                Assert.Fail(ex.ToString());
            }
        })
        );
    }

• Add try/catch on the events to avoid possible unhandled exception that lead to the test host hang.