Use `env::temp_dir()` for BP tests

Use `env::temp_dir()` for BP tests

Currently `BackgroundProcessor` tests create persister directories in the
current working directory and rely on cleaning up in a `Drop` implementation.

Unfortunately, it seems that in the async tests that nodes are not
`drop()`ed for some reason and so the directories created by those
tests remain behind in the current working directory.

This commit at least ensures that these test directories are created in
a temporary location for the OS using `temp_dir()`. It doesn't aim to
solve the lack of cleanup in the async tests.

Partial fix for #2224 but I believe it's enough to resolve it as these
temp directories that do remain will be purged by the OS at some stage
and are overwritten by subsequent tests if there is a conflict.