simulators/eth2: Add test for block value boost flags across clients

[marioevz]

Create a test using the mock builder that verifies the behavior of these flags.

• We should create a hive flag that translates generic value into a client specific value that can be properly parsed by the flag inside the client's docker bash script
• We should test that:
  • a) Even with the flag set to a very high value, the client still uses the builder's payload if the reward indeed surpasses the minimum expected value
  • b) The client correctly falls back to local block production if the builder does not surpasses this value
• The test cannot expect the value to be exact (e.g. on a 10% value, the mock builder is not configured to boost to exactly 10%, because the test will fail intermittently)

Nimbus: status-im/nimbus-eth2#6103
Lighthouse: sigp/lighthouse#5441
Lodestar: ChainSafe/lodestar#6568
Teku: Consensys/teku#8108
Prysm: prysmaticlabs/prysm#13772

@fredriksvantes