Fix Failures with GuiTests `InstallerTest` module

Fix Failures with GuiTests `InstallerTest` module

* If the `...-x64-PerUser-Setup.exe` file is missing from the `Build/` folder tree, then GuiTests
fails with
  a) a system message box suggesting a file name misspelling, plus
  b) seven ASSERT failures that have to be manually Ignored

* This patch allows GuiTests to detect the missing file itself and cause a proper GTEST_FAIL()
action.

* The filename for the Setup.exe file is upgraded from version `2.16.7.0` to `2.16.8` (the latest
publicly available setup file that I could find)