Wpf: Add license headers to test code

[stevenbrix]

[ConnorL33t]

I'll do this, should be pretty quick!

[karelz]

Great! Looking forward to it!
BTW: If it is too big (hundreds of files), I think we would be fine with iterations - each in separate PR. (I didn't check how many files are there ...)

[karelz]

BTW: We have used Code Formatter tool in the past for these things. I'm told it does not work for test projects, but maybe it can be fixed up ... that way you avoid manual work / ad-hoc scripting and it will be useful for other projects in future.