Only override private BLS key in test mode if TestPrivateBlsFlag is defined

[pschork]

Only override private BLS key in test mode if TestPrivateBlsFlag is defined

Previously enabling test mode required TestPrivateBlsFlag to be set or would halt node with

pp-eigenda-native-node  | 2025/02/07 20:45:40 application failed: failed to create BLS signer: Element.SetString failed -> can't parse number into a big.Int

Checks

• I've made sure the tests are passing. Note that there might be a few flaky tests, in that case, please comment that they are not relevant.
• I've checked the new test coverage and the coverage percentage didn't drop.
• Testing Strategy
  • Unit tests
  • Integration tests
  • This PR is not tested :(