enhanced random_string() test helper function
enhanced random_string() test helper function


Former-commit-id: 31a3b7a214021c7181061c8f067df778d434956e [formerly 0ca35aca1bbb754075ebbe4c971803a62bf2b0f9]
Former-commit-id: 09b0fba3757bf5394847bdb62b1eba2c2fa88b14
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine