errors: indent "invalid value for %define" context

errors: indent "invalid value for %define" context

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

For example, what used to be:
  input.y:1.9-29: invalid value for %define variable 'foo' : 'bar'
  input.y:1.9-29: accepted value: 'most'

is now:
  input.y:1.9-29: invalid value for %define variable 'foo' : 'bar'
  input.y:1.9-29:     accepted value: 'most'

* src/muscle-tab.c (muscle_percent_define_check_values): 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>