raft: fix box.ctl.promote() hanging when cannot gather a quorum

Fixing a bug with nodes in 'manual' election mode bumping the term
excessively revealed a hang in election_pre_vote test. Turns out the
test passed thanks to the previous buggy behaviour.

The following behaviour is expected: when a node is configured in manual
election mode, calling box.ctl.promote() on it should make it bump term
once, try to gather votes and fail on timeout.

Once the extra term bump on timeout was removed in commit 5765fdc4
("raft: fix 'manual' nodes bumping the term excessively"),
box.ctl.promote() without a quorum started hanging.

Let's return the correct behaviour: 'manual' nodes should transition
back to follower if an election timeout passes after the promotion
without any term outcome.

Enable the test_promote_no_quorum testcase of election_pre_vote test
back, since it's fixed now.

Follow-up #8168
Closes #8217

NO_DOC=bugfix
NO_CHANGELOG=changes not released behaviour

(cherry picked from commit 352fe0c7)