Wrong amount of docs for test corpora when creating a track with create-track

[dliappis]

As reported in https://discuss.elastic.co/t/why-rally-extract-1001-docs-in-test-mode/282836/3, when we create a track using the create-track subcommand, and the included indices contain >1000docs, we -1k corpora (used by --test-mode) contain 1001 docs.

Replication:

1. Install Elasticsearch

   INST_ID=$(esrally install --quiet --distribution-version=7.14.0 --car=4gheap,basic-license --network-host=127.0.0.1 --http-port=9200 --node-name=es01 --master-nodes=es01 --seed-hosts=127.0.0.1 --runtime-jdk=bundled | jq --raw-output '.["installation-id"]')

2. Start Elasticsearch

   esrally start --installation-id=$INST_ID --race-id=test1234 --runtime-jdk=bundled

3. Load some data

   esrally race --pipeline=benchmark-only --target-hosts=127.0.0.1:9200 --track=geonames --challenge=append-no-conflicts-index-only --track-params="ingest_percentage:0.05" --on-error=abort --include-tasks="delete-index,create-index,check-cluster-health,index-append"

4. Create a new track:

   $ esrally create-track --track=mynewtrack --indices=geonames --target-hosts=127.0.0.1:9200
   
   [INFO] Connected to Elasticsearch cluster [es01] version [7.14.0].
   
   Extracting documents for index [geonames] for test mode...    1001/1000 docs [100.1% done]
   Extracting documents for index [geonames]...                40000/40000 docs [100.0% done]
   
   [INFO] Track mynewtrack has been created. Run it with: esrally --track-path=/.../tracks/mynewtrack
   

5. Observe the output above showing 1001/1000; also verifying using:

   $ wc -l tracks/mynewtrack/geonames-documents-1k.json
   1001 tracks/mynewtrack/geonames-documents-1k.json
   

This is due to off-by-one error in

rally/esrally/tracker/corpus.py

Line 84 in 1b56ef4

if n > total_docs:

. I will raise a PR fixing this shortly.