doc: update test/README.md

take the hardcoded list out of the readme. this way, we only need to
update the script as new tags are added

Author: josibake

test/README.md

@@ -95,12 +95,14 @@ Run all possible tests with
 test/functional/test_runner.py --extended
 ```
 
-In order to run backwards compatibility tests, download the previous node binaries:
+In order to run backwards compatibility tests, first run:
 
 ```
-test/get_previous_releases.py -b v23.0 v22.0 v0.21.0 v0.20.1 v0.19.1 v0.18.1 v0.17.2 v0.16.3 v0.15.2 v0.14.3
+test/get_previous_releases.py -b
 ```
 
+to download the necessary previous release binaries.
+
 By default, up to 4 tests will be run in parallel by test_runner. To specify
 how many jobs to run, append `--jobs=n`