Fix tests with "relative" paths on Windows

Absolute paths on Unix, e.g. /foo/bar, are not absolute on Windows,
which breaks many test cases. This commit adds a method to
PuppetSpec::Files.make_absolute that makes the path absolute in
test cases.

On Unix (Puppet.features.posix?) it is a no-op. On Windows,
(Puppet.features.microsoft_windows?) the drive from the current
working directory is prepended.

Reviewed-by: Jacob Helwig <jacob@puppetlabs.com>