ART: Distinguish Checker lines from comments

In order to prevent tests passing due to lines with hard-to-spot
formatting errors begin ignored, e.g. by forgetting the colon after
"//CHECK", Checker will now require its assertions to start with "///"
or "##", respectivelly for Java and Smali. Such lines will never be
ignored and will fail the test unless successfully parsed.

Change-Id: I0da9a8f13eb96d950af8c85df17d1899a853a299