gunit: Add unit tests for get_next_rule_field()

gunit: Add unit tests for get_next_rule_field()

[----------] 10 tests from GetNextRuleField
[ RUN      ] GetNextRuleField.InvalidParameters
[       OK ] GetNextRuleField.InvalidParameters (0 ms)
[ RUN      ] GetNextRuleField.FieldLenTooSmall
[       OK ] GetNextRuleField.FieldLenTooSmall (0 ms)
[ RUN      ] GetNextRuleField.FieldLenTooSmallWithQuotes
[       OK ] GetNextRuleField.FieldLenTooSmallWithQuotes (0 ms)
[ RUN      ] GetNextRuleField.FieldLenTooSmallWithQuotes2
[       OK ] GetNextRuleField.FieldLenTooSmallWithQuotes2 (0 ms)
[ RUN      ] GetNextRuleField.UserAndProcess
[       OK ] GetNextRuleField.UserAndProcess (0 ms)
[ RUN      ] GetNextRuleField.UserOnly
[       OK ] GetNextRuleField.UserOnly (0 ms)
[ RUN      ] GetNextRuleField.KeyWithQuotesAndSpaces
[       OK ] GetNextRuleField.KeyWithQuotesAndSpaces (0 ms)
[ RUN      ] GetNextRuleField.DestinationWithQuotes
[       OK ] GetNextRuleField.DestinationWithQuotes (0 ms)
[ RUN      ] GetNextRuleField.TabsAsDelimiters
[       OK ] GetNextRuleField.TabsAsDelimiters (0 ms)
[ RUN      ] GetNextRuleField.RuleWithOptions
[       OK ] GetNextRuleField.RuleWithOptions (0 ms)
[----------] 10 tests from GetNextRuleField (0 ms total)

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>