This change makes sure that even if the tests are built in a
"release" configuration (with optimizations and NDEBUG turned on),
the assertions won't get compiled out of the tests themselves.

The C standard guarantees that the most recent inclusion of
<assert.h> is the one that counts, so it's important that the
"#undef NDEBUG/#include <assert.h>" come as the last thing in
the block of header files.

"testutil.hpp" includes <assert.h>, so I've left <assert.h> out
of any test that #includes "testutil.hpp", just for the sake of
brevity.