check_syntax: Fix ignoring commented content

[Lestropie]

As noted in #3237.

Can't just discard everything after a single-line code comment and then perform a check, as a disable directive might be present within the comment.

Perhaps instead of dealing with the disable directive within the regex, discard all lines containing such a directive before executing any regex. Indeed this might permit the performing of these checks after discarding comment blocks.

[Lestropie]

Closed by #3251.