BUG: Fix unit tests single/multi-configuration

Fix incorrect handling of single- versus multi-configuration generators
assuming that we are multi-configuration iff WIN32. Fix missing handling
of multi-configuration generators in recently added configure launcher
tests.
BUG: Fix App4ConfigureLauncherTest-ConfigureStep

Tweak ctkAppLauncher.cmake to accept an optional argument with build
configurations to use as hints when looking for the launcher executable
(which is needed to find it in the build dirextory of a multi-config
build). Adjust test invocation to pass along current configuration to
the test scripts, and aforementioned test to pass along the
configuration when configuring the test project, for use in the new
search path list.

This fixes App4ConfigureLauncherTest-ConfigureStep failing with
multi-config generators.