test: Improve x/evm genesis tests

Previously, the InitGenesis function did not have complete unit
coverage, did not check for panic values, and shared a test suite with
the handler.  This created dependencies on setup for unrelated tests and
made it difficult to add, change, or refactor genesis behavior with
confidence.

These changes use a fixture based approach to testing InitGenesis which
improves the ability to define more complex test cases and run each sub
test in isolation.  Once global state is removed from app setup, this
enables an easier transition to parallel tests.

This comes from the ability to setup the application, define a function
for each test that is provided a context and app, then generates a
fixture with required dependencies and expectations.  This allows
context to be shared across state setup, expectations, and panic values.

In addition, all InitGenesis behavior was checked in TDD fashion and
tests were added where coverage was lacking before.

x/evm/genesis.go

@@ -25,12 +25,29 @@ import (
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/crypto"
 	precompile_modules "github.com/ethereum/go-ethereum/precompile/modules"
+
 	ethermint "github.com/evmos/ethermint/types"
 	"github.com/evmos/ethermint/x/evm/keeper"
 	"github.com/evmos/ethermint/x/evm/types"
 )
 
-// InitGenesis initializes genesis state based on exported genesis
+// InitGenesis initializes genesis state based on the provided genesis state which
+// is either for a new chain, or from an exported existing chain.
+//
+// The context is used to provide context to keeper function calls and to load
+// the EVM Chain ID, which is required by EIP-155.
+//
+// The EVM Keeper is used to normalize and store the provided data or genesis state.
+//
+// The Account Keeper is used to check corresponding accounts exist for the module
+// and EVM genesis accounts.
+//
+// The registered precompiles list is used to ensure that any param enabled precompiles
+// exist and are included in the binary.
+//
+// Since the data provided is assumed to have already passed basic validations,
+// we only directly check stateful validations and assumptions of external state
+// from the Account Keeper and registered precompile list.
 func InitGenesis(
 	ctx sdk.Context,
 	k *keeper.Keeper,
@@ -40,6 +57,8 @@ func InitGenesis(
 ) []abci.ValidatorUpdate {
 	k.WithChainID(ctx)
 
+	// For an enabled precompile to be valid,
+	// it must exist in the binary and be registered
 	err := types.ValidatePrecompileRegistration(
 		registeredModules,
 		data.Params.GetEnabledPrecompiles(),
@@ -61,6 +80,7 @@ func InitGenesis(
 	for _, account := range data.Accounts {
 		address := common.HexToAddress(account.Address)
 		accAddress := sdk.AccAddress(address.Bytes())
+
 		// check that the EVM balance the matches the account balance
 		acc := accountKeeper.GetAccount(ctx, accAddress)
 		if acc == nil {