Rework memory testing (or remove?)

[skirpichev]

In principle, handling of memory failures is tested in the libzz. Though, maybe it does make sense to keep some basic tests here as well, to check that it really works for a Python extension.

But we need to find a better way for this. E.g. current testing affected by the presence of the hypothesis plugin:

$ pytest -q tests/test_memory.py 
...                                                                      [100%]
3 passed in 16.80s
$ pytest -p no:hypothesispytest -q tests/test_memory.py 
...                                                                      [100%]
3 passed in 30.10s

We could run tests in subprocesses instead and also use ulimit (available in MinGW) to set memory/stack limits.

See also #73.