For what I'm doing at the moment I want the tests in the same assembly as the code I'm testing.
I was using version 7.0.0. I uninstalled that and installed 7.0.4 and I can confirm that the issue with the 'ExtensionAttribute defined in multiple assemblies' warning is fixed.
After installing 7.0.4 there were still some issues though. The first couple of times I loaded Visual Studio it displayed the following errors in messageboxes one after the other.
The 'VSTS for Database Professionals Sql Server Data-tier Application' package did not load correctly.
The 'RadLangSvc.Package, RadLangSvc.VS, Version=10.0.0.0, Culture=neutral, PublicKeyTocken=89845dcd8080cc91' package did not load correctly.
The 'Language Package' did not load correctly.
After reloading Visual Studio a few times that went away. Then there was another issue where shortly after opening a solution or adding a new project to a solution Visual Studio reported that an exception had occured. I ran Visual Studio with logging enabled, the log contained the following:
<entry>
<record>367</record>
<time>2012/04/03 10:11:51.290</time>
<type>Error</type>
<source>Editor or Editor Extension</source>
<description>System.NullReferenceException: Object reference not set to an instance of an object.
 at Typemock.Common.Core.Logging.Aspects.ReportsGException.OnException(MethodExecutionArgs eventArgs)
 at Typemock.AutoRun.Adorners.MethodSummaryAdornmentFactory.TextViewCreated(IWpfTextView textView)
 at Microsoft.VisualStudio.Text.Editor.Implementation.WpfTextView.<>c__DisplayClassd.<BindContentTypeSpecificAssets>b__a()
 at Microsoft.VisualStudio.Text.Utilities.GuardedOperations.CallExtensionPoint(Object errorSource, Action call)</description>
</entry>
<entry>
<record>368</record>
<time>2012/04/03 10:11:51.415</time>
<type>Error</type>
<source>Editor or Editor Extension</source>
<description>System.NullReferenceException: Object reference not set to an instance of an object.
 at Typemock.Common.Core.Logging.Aspects.ReportsGException.OnException(MethodExecutionArgs eventArgs)
 at Typemock.AutoRun.Adorners.MethodContextAdornmentFactory.TextViewCreated(IWpfTextView textView)
 at Microsoft.VisualStudio.Text.Editor.Implementation.WpfTextView.<>c__DisplayClassd.<BindContentTypeSpecificAssets>b__a()
 at Microsoft.VisualStudio.Text.Utilities.GuardedOperations.CallExtensionPoint(Object errorSource, Action call)</description>
</entry>
Disabling TypeMock AutoRunner makes this exception go away. This is as before with 7.0.0 where you misunderstood my first post. I wasn't suggesting that the Lint and AutoRunner extensions being disabled was an issue, I was merely pointing out that I'd disabled AutoRunner myself to prevent this exception.