runtests: make random seed fixed for a month

When using randomized features of runtests (-R and --shallow) it is
useful to have a fixed random seed to make sure for example extra
commits in a branch or a rebase won't change the seed that would make
repeated runs work differently.

As it is also useful to change seed sometimes, the default seed is now
determined based on the current month (and first line curl -V
output). When the month changes, so will the random seed.

The specific seed is also shown in the standard test suite top header
and it can be set explictly with the new --seed=[num] option so that the
exact order of a previous run can be achieved.

Closes #4734

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