added "make valgrind" targets to run the test suite under the debugger

added "make valgrind" targets to run the test suite under the debugger

* Makefile.am tests/Makefile.am tests/*/Makefile.am
  tests/*/*/Makefile.am : added "make valgrind" targets
  to run the test suite under the debugger control
* transform.c: valgrind spotted 2 bugs, one related to
  the ordering of the deallocation of the data associated to
  a transofrmation, the second in xsltCopyTree when the new
  node may have been coalesced with an adjacent text node.
  The regression tests now pass cleanly under testgrind.
Daniel