[TEST] fixed error message in REST tests runner
Check ThreadInfo[] for null element if thread are not alive.

If a thread is not alive getting ThreadMXBean#getThreadInfo(long[], int)
places null elemnents in the returned array which are not repected
in the HotTheards API.

Closes #4775
[SPEC] Separated info.json into info and ping.json (GET vs HEAD)
[SPEC] Fixed bad name in ping.json
[TEST] Removed TODO and custom code now that ping and info are two different apis in the REST spec
[SPEC] Removed _source endpoint from get api spec, as the get_source is a different api already

Also removed custom code that was required in the REST tests due to this.