Make files non-executable

@FroMage somehow made one file non-executable in 7eb3611, so I checked
if we had any more executable files around:

    find -perm 755 -type f -exec chmod -x {} +

And apparently we have LOADS of them. Most of them in test-samples,
where I create new files by copying existing ones (test-samples isn’t a
source folder, so the Ceylon explorer refuses to create new Ceylon
source files there, making copy+paste the easiest method to create new
files); either one file was executable for some reason, and the
executable bit spread from there via copy+paste, or perhaps the IDE sets
the executable bit on copied files for some reason?

(I manually re-chmodded the ceylon-format script.)