Fix service enable specs on platforms without enable

These specs use the default service provider, but in the case where that
provider doesn't support the enable property, they were failing when
testing various values for that property. Many of these went unnoticed
because they weren't actually making any assertions, but rather checking
that nothing failed, so now they do assert.
Use the right provider instance in launchd provider specs

These tests were creating a resource and a launchd provider instance
from that resource, when the resource already has a (default) provider instance.
In most cases this was working fine, because the resource's provider
instance wasn't often being referred to. However, a couple of tests were
trying to set the enable property of the resource, which fails on systems without an
enableable provider. Now, we explicitly set the resource's provider to
launchd, as use its provider instance for all of our tests.
Fix posix exec provider spec on systems with /bin/cd

This test was setting path to /bin with the assumption that it didn't
contain cd, and therefore the only cd would be a shell built-in. On
system which *do* contain /bin/cd, this was failing. Now we just use a
bogus path so this will work everywhere.
Skip test that runs git if we can't run git

This test was assuming, incorrectly, that it will always be run from
inside a git repo with git installed. Now we properly handle the case
where it's not, and skip the test.
Merge pull request #1408 from nicklewis/aix-specs

(AIX) Fix some failing AIX specs