asim: use one dedicated generated directory per non_rand subtest and clear it with -rewrite

[This comment](https://github.com/cockroachdb/cockroach/pull/153991#pullrequestreview-3262240595) notes that it might be better if instead of mixing multiple subtests (mma-count, sma-count, etc) within one generated artifacts directory, we should give each one their own. Then we can also clear out the directory if we're about to rewrite it, without having to worry about deleting files from another subtest (which may not be regenerated right now due to a test filter).
We'd probably (or at least plausibly) need to update `asimview` to understand this enhanced directory structure, depending on how it's written. Shouldn't be a big deal, though.

Epic CRDB-56265
Jira issue: CRDB-54718