test(config): pin the dotted account key for startup_account

startup_account matches Account::key, the [account.<key>] suffix, and not the
mbsync channel. The two genuinely differ in a real setup: a section key may
carry dots that the channel does not.

The dot is the part worth pinning. QSettings treats "/" as a group separator,
which is why account sections use a dot at all; a dot INSIDE the key is a
different case, and a silent mismatch would report "not a configured account"
and quietly start on All accounts. Verified against a standalone probe before
this test was written.

The second half asserts the channel is rejected, so the two names cannot be
interchangeable in one direction only.

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