tests: adjust the new, very expensive rm test to be less expensive

* tests/rm/4-million-entry-dir: Create only 200,000 files, rather
than 4 million.  The latter was overkill, and was too likely to
fail due to inode exhaustion.  Not everyone is using btrfs yet.
Now that this test doesn't take so long, label it as merely
"expensive", rather than "very expensive".  Thanks to
Bernhard Voelker for pointing out the risk of inode exhaustion.

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