Repair appveyor... Horrifyingly

It's sort of gnarly that it's still doing in-tree builds, but I
really, _really_ don't want to get any more friendly with CMake's
Visual Studio generator to work out how to make it stop doing it.

In-tree builds still work, after all, and the goal of this work is
to make out-of-tree builds work as well. Notional horrors like
this will have to wait ;)

Author: ChrisKitching

appveyor.yml

@@ -7,5 +7,5 @@ build_script:
 - cmake . -G "Visual Studio 14 2015"
 - cmake --build . --config Release
 test_script:
-- Release\json_unit.exe
-- Release\json_unit.exe "*"
+- test\Release\json_unit.exe
+- test\Release\json_unit.exe "*"