Put CU_TEST_NAME in the environment for each test run by "make check".

Put CU_TEST_NAME in the environment for each test run by "make check".

* Transform all Makefile.am files so that when running "make check",
CU_TEST_NAME is set to the name of the test.  This is so that when I
run valgrind-enabled (--log-file-qualifier=CU_TEST_NAME) "make check"
on the entire package it is more convenient to map a leak or error
found in a valgrind log file back to the offending test.
Use this command:
(echo tests/Makefile.am.in; find tests -name Makefile.am) \
 |xargs perl -pi -e '/^(\s*)PATH=...VG_PATH_PREFIX/ and ' \
   -e 'print $1,q|CU_TEST_NAME=`basename $(abs_srcdir)`,$$tst |,"\\\n"'