fad798be76 test: Default --previous-releases to false if dir is empty (MarcoFalke)
faf1c3cc58 test: Replace TEST_PREVIOUS_RELEASES env var with test_framework option (MarcoFalke)
Pull request description:
The "auto-detection" feature is kept in place, but making it an option allows to properly document it. For example, on my machine I get:
```
$ ./test/functional/wallet_disable.py --help | grep previous-releases
--previous-releases Force test of previous releases (default: False)
ACKs for top commit:
Sjors:
re-tACK fad798b
Tree-SHA512: a7377d0d5378be0a50be278d76396cc403583617b5fc43467773eee706df698acf3f4e67651491183b9b43a8e1816b052e4c17b90272b7ec4b6ac134ad811400