changed ModuleHeaderTest.cmake to install headers in case build_testing=...

...off

fixes #160 

basically removed:
if( NOT ${BUILD_TESTING} )
   return()

with
if( ${BUILD_TESTING} )
and do nothing in case BUILD_TESTING=OFF

I am not sure that I understand it. The headers should be installed in any case, right? So it should not be outside the if?
@arnaudgelas Could you have a quick look at it? I don't really know what was intended.