The assertion ensuring that there were at least 3 metrics received
didn't end up being useful. If this assertion fails, it doesn't explain
what segment events _were_ received.
By removing this assertion and letting the later assertions catch this
case, we at least learn which of the three expected events were
present.
An e2e test has been added that uses the new mock Segment server to
verify that the three initial page metric events are sent correctly.
Using the mock Segment server requires a special build with this mock
Segment server hostname embedded, so a distinct job for building and
running this test was required. As such, it was left out of the
`run-all.sh` script.
