Never thought I'd be posting VB to my blog, but here's a little Visual Studio macro to switch between a Class file and it's associated TestFixture. The macro assumes a ClassNameFixture naming convention. Also sorry if my VB sucks, it literally took me 3 minutes plus asking a coworker to figure out how test for null.
Enjoy:
Imports System
Imports System.IO
Imports EnvDTE
Imports EnvDTE80
Imports System.Diagnostics
Public Module TestFixtures
    Public Sub SwitchBetweenClassAndFixture()
        Dim active As Document, filename As String, extension As String
        Const fixtureSuffix = "Fixture"
        active = DTE.ActiveDocument
        If Not active Is Nothing Then
            filename = Path.GetFileNameWithoutExtension(active.Name)
            extension = Path.GetExtension(active.Name)
            If filename.EndsWith(fixtureSuffix) Then
                DTE.ExecuteCommand("Edit.OpenFile", filename.Substring(0, filename.Length - fixtureSuffix.Length) + extension)
            Else
                DTE.ExecuteCommand("Edit.OpenFile", filename + fixtureSuffix + extension)
            End If
        End If
    End Sub
End Module
