os_regex unit tests

adding more unit tests for os_regex

p.s.:
the regex extraction tests is crashing for me, cause os_regex is trying to modify the const input strings (https://github.com/ossec/ossec-hids/blob/master/src/os_regex/os_regex_execute.c#L72).
I think i fixed this in my branch os_regex(https://github.com/cgzones/ossec-hids/tree/os_regex).