Improve Verilator compatibility 2

[flooklab]

In tests, e.g. https://github.com/SiLab-Bonn/basil/blob/master/tests/test_SimFifo8to32.py,
empty may not settle correctly with Verilator, so that tests will fail.
This is solved by adding reset for empty.

[codecov]

Codecov Report

Merging #149 (e517fca) into master (af5e949) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   31.92%   31.92%           
=======================================
  Files          72       72           
  Lines        4780     4780           
=======================================
  Hits         1526     1526           
  Misses       3254     3254           

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