(fact-273) Return a string for stub of :kernel fact

Previous to this change, this spec test was stubbing :kernel
as :windows. But now facter a) does type-checking on fact
values (Symbol is not allowed) and b) warns on type-check
errors.

Changing this stub to return 'windows' reduces warning
messages during spec runs.
(fact-273) Expect a warning message that can be expected

Add a warning message expectation to suppress the warning message
on stdout/stderr during spec runs.  Also, remove a stub that
duplicated a stub in the before block.
(fact-273) Remove a test which was a subset of the following test
(fact-273) Add expectation for Facter.warn on timeouts
(fact-273) Add expectation for Facter.warn on exceptions retrieving operatingsystemrelease
(fact-273) Add expectation for Facter.warn on ignored file extensions
(fact-273) Add expectation for Facter.warn on resolutions that throw an exception
Merge pull request #627 from kylog/issue/fact-273/address-warnings-in-spec-runs

Issue/fact 273/address warnings in spec runs