check_mem_leaks: add new unit test for use against valgrind

check_mem_leaks: add new unit test for use against valgrind

The purpose of this test is to be used by valgrind to check for
memory leaks. Each public API that check exports is used at
least once. Tests which use non-public API, or leak intentionally,
are not included here.

The test should pass when run, and is added to the list of tests to
run with "make check".

Actually using valgrind to check for memory leaks is outside of its
running with "make check". One still would need to manually run
valgrind.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@901 64e312b2-a51f-0410-8e61-82d0ca0eb02a