Previously, the cached `tempdir` value was stored in `common.state`.
Unlike the other `common.state` values, this isn't immediately useful to
other commands (they can just call the tempdir API). So, this moves the
cached value into `tempdir.js`.
This also adds a unit test for the caching behavior, and exposes
test-only helpers to verify this behavior.
Finally, this adds a note to `common.state` that values should generally
be considered read-only, since this can be important for customized
behavior. Although, I recognize our code base has one exception to this
rule (`echo()`), we should strive to maintain this.
Fixes#902
Test: Added a unit test.
6b3c7b1e44