tests: Make robust against variations in PRNG use

tests: Make robust against variations in PRNG use

Tests need repeatable pseudorandom numbers to yield repeatable
results.  We seed the pseudorandom number generator with a fixed value
(emp_server -R) to make it produce the same sequence of numbers every
time.  But whenever code exercised by a test is changed to consume
fewer or more of them, all subsequent users get different numbers
regardless.  The ensuing test result changes are extremely tedious to
review.

To address this problem, reseed the PRNG with the count of commands
right before executing a command when running_test_suite is on.  This
way, the effect of perturbing the PRN sequence lasts only until the
next command.

Note that the next command could be another player's.  Doesn't matter.

Adding or removing commands now upsets the PRN sequence even for
commands that don't consume PRNs.  The next commit will take care of
that.

Perturbs test results across the board.  Hopefully, that'll happen
much less frequently now.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>