tests: fix/optimise download timeout spread test

The download-timeout test was taking over 10min to complete due to a couple of
issues. The first one is that there was a typo in the environment variables
that controlled the measurement window so the timeout was still the default of
5min (the test downloads twice). Setting it correctly cut the execution time
for one of the downloads but not the other strangely. It turns out that
the test was creating a override file that was sorted before the other
existing override (systemd applies env vars in order).  Fixing this and
lowering the timeout to 15s cuts the running time from 10min to 30s. I wanted
to add a log when the env var was set so we could check it in the test but
this happens in an `init()` and I don't think the logger is set up by then.

Signed-off-by: Miguel Pires <[email protected]>

tests/main/download-timeout/task.yaml

@@ -9,22 +9,20 @@ systems: [ubuntu-18.04*, ubuntu-2*]
 
 environment:
   # set minimum download speed that is a bit higher than the rate-limited speed set with tc.
-  # also force 1-minute window, so the test doesn't have to run for full 5 minutes (the default
+  # also force a 15s window, so the test doesn't have to run for full 5 minutes (the default
   # window).
-  SNAP_MIN_DOWNLOAD_SPEED: 99000
-  SNAP_DOWNLOAD_MEAS_WINDOW: 1m
-  OVERRIDES_FILE: /etc/systemd/system/snapd.service.d/env-overrides.conf
+  SNAPD_MIN_DOWNLOAD_SPEED: 99000
+  SNAPD_DOWNLOAD_MEAS_WINDOW: 15s
+  OVERRIDES_FILE: /etc/systemd/system/snapd.service.d/local.conf
 
 prepare: |
   if not os.query is-pc-amd64; then
       echo "tc fail to add a device in arm architecture with error: 'Cannot find device ens4'"
       exit
   fi
 
-  cat <<EOF >> "$OVERRIDES_FILE"
-  Environment=SNAP_MIN_DOWNLOAD_SPEED=${SNAP_MIN_DOWNLOAD_SPEED}
-  Environment=SNAP_DOWNLOAD_MEAS_WINDOW=${SNAP_DOWNLOAD_MEAS_WINDOW}
-  EOF
+  cp "$OVERRIDES_FILE" "$OVERRIDES_FILE".bak
+  sed "s/Environment=/Environment=SNAPD_MIN_DOWNLOAD_SPEED=${SNAPD_MIN_DOWNLOAD_SPEED} SNAPD_DOWNLOAD_MEAS_WINDOW=${SNAPD_DOWNLOAD_MEAS_WINDOW} /" -i "$OVERRIDES_FILE"
 
   systemctl daemon-reload
   systemctl restart snapd.{socket,service}
@@ -42,7 +40,7 @@ restore: |
   fi
   tc qdisc del dev ens4 ingress
 
-  rm -f "$OVERRIDES_FILE"
+  mv "$OVERRIDES_FILE".bak "$OVERRIDES_FILE"
   systemctl daemon-reload
   systemctl restart snapd.{socket,service}