Also update BUILD.txt

The leak checking code which is used by the resolve and failover tests
frees talloc's autofree context which is not recommended. As a
consequence the initialization of tevent failed when it was called by
the second test and CF_FORK=no, because it holds some data in the
autofree context.

This patch introduces a global talloc context which should be uses by
the test as the root of their memory hierarchy instead of NULL. This
global context is used in the leak checking routines.

Not all types of memory leaks can be detected by the new version , it is
recommended to use valgrind or similar tools additionally.

The leak_check_setup() and leak_check_teardown() functions can be added
to a test case with tcase_add_checked_fixture(). They will make sure
that all tests are checked for memory leaks. However, since talloc is
hierarchical and automatically frees the children, this will not catch
all cases, but might still be helpful.

The check_leaks(ctx, bytes) function takes a talloc context as an
argument and the number of bytes it should be using up (children
included). The total byte size used up by the context is determined by
the talloc_total_size() function. If the size doesn't agree,
check_leaks() will print out a talloc report on the context and makes
the current test fail.

The check_leaks_push() and check_leaks_pop() both take a talloc context
as an argument. Every time push is called, the context is "pushed" onto
an internal stack and it's current size is noted. When the context is
later "poped", the pop function will make sure that the size is the same
as when it was pushed. It will also check that it's not called
out-of-order or if the stack isn't empty.