[zstreamtest] Reduce memory of newapi tests

[zstreamtest] Reduce memory of newapi tests

We could allocate up to 2^28 bytes of memory when using 2 threads with
window log = 24. Now, we limit it to 2^26 bytes of memory when not running
big tests.

I chose max window log = 22 since that is the maximum source size when
big tests are disabled. Hopefully this will be enough to reduce or
eliminate the test failures.