Wait for exec before snapshot

[thomasjpfan]

This PR fixes the snapshotFilesystem flaky test by waiting on the exec.

[cursor]

PR Summary

Low Risk
Low risk test-only change that reduces flakiness by ensuring sb.exec() commands finish before taking a snapshot; no production logic is modified.

Overview
Fixes a flaky snapshotFilesystem test by explicitly waiting for the file-write and directory-create sb.exec() processes to complete before calling sb.snapshotFilesystem().

This changes the test to capture exec handles (writeFile, mkDir) and await ...wait() on each, ensuring the subsequent snapshot reliably contains the expected filesystem state.

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[ehdr]

LGTM!

I think there are a bunch more .exec that we don't .wait for (also in Go?), but can fix those separately too if you prefer.

[thomasjpfan]

I think there are a bunch more .exec that we don't .wait for (also in Go?)

Yea, I prefer to follow up fixing it. I've seen this specific snapshot test fail often enough on main + PRs.