Currently, we pass the `pickleKey` to the rust library for use as a passphrase
for encrypting its crypto store. The Rust libary then passes that passphrase
through 200000 rounds of PBKDF2 to generate an encryption key, which is
(deliberately) slow.

However, the pickleKey is actually 32 bytes of random data (base64-encoded). By
passing the raw key into the rust library, we can therefore save the PBKDF
operation.

Backwards-compatibility with existing sessions is maintained, because if the
rust library discovers that the store was previously encrypted with a key based
on a PBKDF, it will re-base64 and PBKDF the key we provide, thus reconstructing
the right key.

Co-authored-by: Florian Duros <florianduros@element.io>

Rely less on the unit under test for setting up the test preconditions -- not
least because we don't really want to fire up matrix clients and the like
during test setup.

For a start it makes it easier to grok what's going on, but also I went to use
this in a test