testutil/beaconmock: fix fuzz issues

[dB2510]

Fixes panics found by initial beacon fuzz tests. Also improve efficiency of beacon fuzzer to trigger more panics in charon workflow.

category: test
ticket: #1962

[codecov]

Codecov Report

Patch coverage: 4.85% and project coverage change: -0.45 ⚠️

Comparison is base (0747a71) 53.74% compared to head (c80bd8b) 53.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2105      +/-   ##
==========================================
- Coverage   53.74%   53.30%   -0.45%     
==========================================
  Files         172      171       -1     
  Lines       22429    22344      -85     
==========================================
- Hits        12054    11910     -144     
- Misses       8899     8977      +78     
+ Partials     1476     1457      -19     

Impacted Files	Coverage Δ
core/tracker/incldelay.go	52.94% <0.00%> (-2.45%)	⬇️
core/validatorapi/validatorapi.go	57.35% <0.00%> (-0.95%)	⬇️
testutil/beaconmock/beaconmock_fuzz.go	0.00% <0.00%> (ø)
core/scheduler/scheduler.go	75.55% <20.00%> (-1.90%)	⬇️
app/app.go	7.80% <40.00%> (-0.02%)	⬇️

... and 9 files with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.