tests: add valgrind support to "make check"

tests: add valgrind support to "make check"

* m4/ax_valgrind_check.m4: Import from the autoconf-archive package.
* configure.ac: Invoke AX_VALGRIND_CHECK.
* valgrind.supp: Move to tests/strace.supp.
* Makefile.am (EXTRA_DIST): Remove valgrind.supp.
* tests/Makefile.am: Add @VALGRIND_CHECK_RULES@.
(VALGRIND_FLAGS, VALGRIND_SUPPRESSIONS_FILES): New variables.
(EXTRA_DIST): Add strace.supp.
* tests/init.sh: Add valgrind command prefix to $STRACE when appropriate.