For policy-specific tests, use the version before the policy was
introduced.  Otherwise, use 3.5 where possible.

Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.

Visual Studio 2022 17 Preview introduced a native ARM64 toolchain.

The RunCMake.GeneratorToolset test expected platform toolsets to have a
name beginning with 'v' followed by one or more decimal digits, as all
the Microsoft platform toolsets follow that naming convention.
The Intel platform toolsets begin with "Intel" and have various
additional strings depending on the compiler version.

This change accepts the toolsets delivered by Intel in addition to those
from Microsoft.

Signed-off-by: William R. Dieter <william.r.dieter@intel.com>

Fixes: #22339

The names and formats of our VS flag tables are internal implementation
details.  However, some institutions need to maintain support for
non-public VS platforms and toolsets.  Provide a hook that their
projects can use to load custom flag table files.  This helps avoid
distributing a custom CMake package within such institutions.

Document the hook itself, but explicitly specify that the files the
hook loads are not considered a stable interface.

Fixes: #21170

Do not rely on the RunCMake infrastructure to pass `-T` or `-A`.

Provide an option to switch back to the original build system via
`-T buildsystem=1`.

Fixes: #18088