[qa] rearrange assertion in wallet-accounts

Summary:
Previously, this test sent the mined fee in order to remove it from
the default account. However, if this fee is less than the dust_limit
then sendfrom will fail. This commit rearranges the assertions so
that sending an amount below the dust threshold is unnecessary.

Test Plan:

./test/functional/test_runner.py wallet-accounts

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1799