errors: indent "%define var" redefinition context

errors: indent "%define var" redefinition context

This is the continuation of the work on the readability of errors
context.

For example, what used to be:
  input.y:2.9-11: %define variable 'var' redefined
  input.y:1.9-11: previous definition

is now:
  input.y:2.9-11: %define variable 'var' redefined
  input.y:1.9-11:     previous definition

* src/muscle-tab.c (muscle_percent_define_insert): Use
complain_at_indent to output with increased indentation level.
* tests/input.at: Apply this change.

Signed-off-by: Akim Demaille <akim@lrde.epita.fr>