spf: add a few tests

TXT record handling will (supposedly) be changed in 0.11, and the SPF breaking change removed in 0.10.27.

Running tests:  [ 'tests/plugins/spf.js' ]

spf.js
 return_results - result, none
 return_results - result, neutral
 return_results - result, pass
 return_results - result, softfail, reject=false
 return_results - result, softfail, reject=true
 return_results - result, fail, reject=false
 return_results - result, fail, reject=true
 return_results - result, temperror, reject=false
 return_results - result, temperror, reject=true
 return_results - result, permerror, reject=false
 return_results - result, permerror, reject=true
 return_results - result, unknown
 hook_mail - rfc1918
 hook_mail - no txn
 hook_mail - txn, no helo

OK: 15 assertions (158ms)
Merge pull request #531 from msimerson/spf

spf: added tests