Update certificate spec tests for Windows

Disable CA related spec tests on Windows, since that functionality is not supported.

Some cert spec tests are still marked as fails_on_windows because
settings attempts to create and apply a catalog corresponding to
ssl/cert related directories. This fails because on Windows
Puppet.features.root? always returns true (which is a separate bug),
and as a result attempts to set the owner and group, which fails because
the provider is not implemented yet on Windows.

Also many of these tests were using Tempfile.new and trying to
system("rm -rf ...") later. I changed these to use
PuppetSpec::Files.tmpdir instead, which automatically cleans up
temporary directories after the tests have run.

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