Add some makefile targets, which use valgrind's memcheck tool to
run the tests, in order to help diagnose memory problems in the
library.

In addition, we enable the '--leak-check' option to report on any
memory leaks. However, unlike the other memory problems reported
by memcheck, memory leak reports do not result in an error exit
from valgrind. (So memory leaks are reported on stderr, but don't
halt the test run.)

A suppressions file (tests.supp) is included since libz triggers
some false positives.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>