Update the dev process wiki to describe how to write tests for zcash_client_sqlite migrations

[str4d]

In fe5c33e a migration test is converted from "create a pre-migration state, test the migration changes it appropriately" to "test that the correct post-migration state is created". However, in general we will want to have both kinds of tests, particularly when fixing bugs with migrations:

• A test to ensure that a new wallet doesn't have the bug.
• A test to ensure that an existing wallet that suffers from the bug is fixed by the migration.

The second kind of test is however only most useful up until the next public release, after which we can generally remove it. And indeed in the commit above, we are removing the "text existing" test because the infrastructure requred to create the broken state is changing sufficiently that we can no longer do so. That being said, if we add a "golden test" framework (#1676) then we'd only need the ability to generate the broken state once, and could then reuse it indefinitely.

We should add a section to https://github.com/zcash/librustzcash/wiki describing the general process for this.