Change assert_* to expect_* in tests

```
grep -Irl assert_ test/ | xargs sed -i \
    's/witness_assert/witness_do_not_replace/g';
grep -Irl assert_ test/ | xargs sed -i \
    's/malloc_mutex_assert_owner/malloc_mutex_do_not_replace_owner/g';

grep -Ir assert_ test/ | grep -o "[_a-zA-Z]*assert_[_a-zA-Z]*" | \
    grep -v "^assert_"; # confirm no output
grep -Irl assert_ test/ | xargs sed -i 's/assert_/expect_/g';

grep -Irl witness_do_not_replace test/ | xargs sed -i \
    's/witness_do_not_replace/witness_assert/g';
grep -Irl malloc_mutex_do_not_replace_owner test/ | xargs sed -i \
    's/malloc_mutex_do_not_replace_owner/malloc_mutex_assert_owner/g';
```

test/include/test/btalloc.h

@@ -25,6 +25,6 @@ btalloc_##n(size_t size, unsigned bits) {				\
 		}							\
 	}								\
 	/* Intentionally sabotage tail call optimization. */		\
-	assert_ptr_not_null(p, "Unexpected mallocx() failure");		\
+	expect_ptr_not_null(p, "Unexpected mallocx() failure");		\
 	return p;							\
 }