lib/lib-test: restore DOVECOT_SRAND feature in DEBUG builds

lib/lib-test: restore DOVECOT_SRAND feature in DEBUG builds

Add a deterministic PRNG, an ability to force its use, and an
ability to re-use the same sequence later.

Since proper random numbers have been forced into use, making
reproducable tests isn't quite as easy as it used to be, it's 3 steps
rather than 2. When seeing an intermittent test failure:
 - rerun the tests with environmental variable DOVECOT_SRAND=kiss
 - upon seeing a new failure case, note the seed logged at the failure
 - debug using DOVECOT_SRAND=<that number>

In non-DEBUG builds, there's no trace of this code, and the
randomisation that is an inherent part of many tests remains
non-reproduceable.

Works with all of the RNG preferences, getrandom/urandom/arc4.

Signed-off-by: Phil Carmody <phil@dovecot.fi>