runtests: introduce --shallow to reduce huge torture tests

When set, shallow mode limits runtests -t to make no more than NUM fails
per test case. If more are found, it will randomly discard entries until
the number is right. The random seed can also be set.

This is particularly useful when running MANY tests as then most torture
failures will already fail the same functions over and over and make the
total operation painfully tedious.

Closes #4699

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