Whitespaces are not well kept in XML: avoid tests using them

Tabulations, double-spaces and line feeds can be replaced by a single
space
Corrected edef_io for Python 2.6 / ElementTree 1.2

ElementTree 1.2.x misses some features used in EDEFWriter.
Those features are added when the module is loaded.
Also, the ElementTree module used in edef_io is removed from sys.modules
before it is modified, so that it doesn't modifies the behavior of the module
for the whole application.