[entropy_src_ast_rng_req_test] Wait 6ms for entropy to become available
[entropy_src_ast_rng_req_test] Wait 6ms for entropy to become available

in a verilated model of OpenTitan the ICache makes the code check for
entropy sooner and have to wait longer before it's available to this
test about 4ms later.

Signed-off-by: Drew Macrae <32d48ac45113625c0a04afe7343eae478a303684@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan