Closes#952
A pragmatic fix for #952: since we already set the server id in `wait_until_ready`,
let's start a test server without an ID (by not passing `INFLUXDB_IOX_ID`)
and use the property of already having an ID as an indication that we're talking
to a server instance that we didn't just start.
It doesn't necessarily mean we're talking to the right server, but the main point
of #952 was to avoid confusing error messages like "DatabaseAlreadyExists";
with this PR, the only way for that error to confuse developers is if we "unset" the
writer ID of a server fixture and leave it there hanging, with in-memory side effects
but no ID. Possible but unlikely, I think.
* refactor: Move test server fixture into its own module
* fix: Update tests/end-to-end.rs
* fix: better error handling and display
* fix: tweak startup message