Add system test which randomly selects $DEFAULT_ALGORITHM

Currently, bin/tests/system/conf.sh.common sets DEFAULT_ALGORITHM for testing to ECDSAP256SHA256 (other variables like DEFAULT_ALGORITHM_NUMBER, DEFAULT_BITS, ALTERNATIVE_ALGORITHM, DISABLED_ALGORITHM, DEFAULT_HMAC, etc. follow from this). We should have a daily CI job which randomly - on runtime - sets DEFAULT_ALGORITHM et al. from algorithms allowed on the current system to ensure that system test continues to work for all supported algorithms where possible.

(Requested by @marka on Friday.)

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