tests: Replace calls to wait_for with assert_eventually

This commit replaces long calls to

  assert_success wait_for 'something to happen' '10 seconds'

with a simple:

  assert_eventually 'something to happen'

if timeout is less than 15 seconds and doesn't really matter.

It also replaces such assertions with a non-standard timeouts and bare
calls to wait_for (to improve error information) there with proper calls
to assert_eventually.

Change-Id: I6db1c761336ad5daa052a7ac002eab7a73adbeae