Detect whether sysnative filesystem alias is available

Previously the test assumed that the sysnative filesystem alias was
available, so that we could run the 64-bit version of reg.exe. However,
this fails when running on x86 versions of Windows. It also fails when
running on x64 on 2003, as the sysnative alias is not available until
Vista and later.

This commit logs a warning if the sysnative alias is not available on
x64 systems. Otherwise, it uses the 'native' version of reg.exe to
check for the default registry value.
Exclude 32: keys and values on 32-bit OS

Previously, the tests would try to create both 32 and 64-bit versions of
keys/values, but on 32-bit OS, those are the same resource, causing our
expectations to fail.

This commit ensure we only try to manage the 32-bit redirected view of
the registry on 64-bit OS.