testutil/keystore: use hardcoded password

[corverroos]

Use hardcoded "simnet" password for keystores since VCs like lighthouse do not support empty passwords when importing keystores.

category: feature
ticket: #251

[codecov]

Codecov Report

Merging #258 (b6da27f) into main (dc35136) will increase coverage by 0.70%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #258      +/-   ##
==========================================
+ Coverage   55.08%   55.78%   +0.70%     
==========================================
  Files          51       53       +2     
  Lines        3571     3671     +100     
==========================================
+ Hits         1967     2048      +81     
- Misses       1327     1345      +18     
- Partials      277      278       +1     

Impacted Files	Coverage Δ
testutil/keystore/keystore.go	50.68% <100.00%> (ø)
core/aggsigdb/memory.go	84.04% <0.00%> (ø)
core/aggsigdb/stub.go	0.00% <0.00%> (ø)
core/types.go	4.25% <0.00%> (+4.25%)	⬆️

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[dB2510]

LGTM!