test: fix three tests to work under bullseye

This patch introduces couple of fixes in our unit tests, which makes the code compile and pass tests when using bullseye code coverage tool.
If there is more than one pool file provided to the test, and the size of the first one is >1GB, but the next is <1GB, then `mprotect()` may fail for the second pool. This is because it checks the `min_addr` and `max_addr` (line #123), which are never reset to 0.
So, either reset `min_addr` and `max_addr` in `munmap()` (line #102), or add the additional condition to line #123:
```
if (mapped_addr != NULL && ...
```

(same in `blk_rw.c`).