I recently had to tackle something similar and got really stumped on the fact that wildcards don't work within the LF:LOOKIN= search - it would be so much simpler if it could, but I haven't found anything indicating that it can.

In my case, I was working in LFWorkflow (and I assume you are as well based on the tags on your post), and that opens up being able to use tokens from earlier activities.

I first found all the top-level folders I needed to search within.  In your example, this should do it:   {LF:LOOKIN="Dev\Sample Documents"}&{LF:Name="Testing", Type="F"}

Then do a "For Each Entry" on the search results.  Within the "For Each Entry", do another search that searches based on the token path of the current entry - something like this:   {LF:LOOKIN="%(ForEachEntry_CurrentEntry_FullPath)"}