Fix posix exec provider spec failures on Windows

These specs were assuming that paths such as /foo were always absolute, which
is not the case on Windows. Thus, when run on Windows, the provider was
complaining about receiving relative paths when it expected absolute, rather
than succeeding or failing in the intended way. Now we expand all paths we want
to be absolute, to guarantee they will be absolute everywhere.

Also, some specs were failing because they were trying to test the case where a
file isn't executable. That's not something we can reliably check on Windows,
so instead just stub the appropriate executable? methods.

Reviewed-By: Matt Robinson <matt@puppetlabs.com>