maint: cleanup code in BIST tests to use Octave coding conventions. * data.cc: Use keyword "endfor" rather than bare "end". * file-io.cc, graphics.cc: Remove semicolon ';' at end of "end_try_catch" keyword. * variables.cc, bug-35448.tst, bug-53027.tst, bug-53579.tst, bug-53599.tst, bug-53956.tst, bug-55758.tst, global.tst: Use semicolon ';' after "global" variable declaration. * for.tst, io.tst: Delete semicolon ';' at end of "for" loop declaration. * struct.tst: Delete semicolon ';' at end of "function" declaration.

%!function rval = sggval (val)
%!  global gval;
%!  if (nargin == 1)
%!    gval = val;
%!  else
%!    rval = gval;
%!  endif
%!endfunction

%!test <*53599>
%! global gval;
%! assert (isempty (gval))
%! sggval (13);
%! assert (sggval (), 13);
%! assert (gval, 13);
%! clear global gval
%! assert (sggval (), [])
%! gval = 42;
%! assert (sggval (), []);
%! clear gval
%! global gval;
%! gval = 42;
%! assert (sggval (), 42);
%! clear -global gval;  # cleanup after test