Fix 2 bugs with Language Standard Feature

[MAJigsaw77]

• Before it was implemented, we were able to set HXCPP_CPP17 inside a Build.xml but in the latest commit we cannot unless we define it inside the command, this behaviour is now fixed by moving the check inside createFileGroup, it allows the xml to be parsed first and then the set for the HXCPP_CPP17 would work just fine.
• Setting the language standard on files element doesnt work as expected, if theres a section element inside the files element it, in that section the language standard that has been set my the parent will be forgotten, this behaviour is now fixed and allows sections to have the language standard work properly.

[tobil4sk]

@Simn This addresses some regressions from #1301, so would be nice to get this looked at. Tests included!

[Simn]

Do we know what's going on with the hanging tests?

[tobil4sk]

I haven't seen them reported anywhere yet, but I've just tested and they happen on master too so unrelated to this PR

[Simn]

That didn't happen on the latest master run so it must come from a Haxe update. Which isn't surprising considering the recent amount of Haxe updates!