Use HasSuffix instead of trying to manually calculate test suffix presence.

[eldondevat]

Can get an out-of-bounds exception at the end of go/database/sql/connection.go if sqlcommenter is asked to comment an empty line/zero-length sql string/comment.

[eldondevat]

@duffn Any interest here? Seems that this can happen sometimes in the context of commenting migrations run by https://github.com/rubenv/sql-migrate