Implement generator toolset selection (cmake -T) for VS >= 10 by setting
the PlatformToolset.  Extend the RunCMake.GeneratorToolset test case to
verify CMAKE_GENERATOR_TOOLSET when the generator supports -T.

Since commit 485a940e (VS: Simplify MSVC version reporting, 2012-08-23)
all MSVC version information is detected during the compiler id step
from the actual compiler invoked by generated build systems rather than
hard-coded in VS generators.  Therefore we can set the PlatformToolset
in VS >= 10 project files and support toolsets from other VS versions.