Add BeaconChainHarness tests for The Merge (#2661)

* Start adding merge tests

* Expose MockExecutionLayer

* Add mock_execution_layer to BeaconChainHarness

* Progress with merge test

* Return more detailed errors with gas limit issues

* Use a better gas limit in block gen

* Ensure TTD is met in block gen

* Fix basic_merge tests

* Start geth testing

* Fix conflicts after rebase

* Remove geth tests

* Improve merge test

* Address clippy lints

* Make pow block gen a pure function

* Add working new test, breaking existing test

* Fix test names

* Add should_panic

* Don't run merge tests in debug

* Detect a tokio runtime when starting MockServer

* Fix clippy lint, include merge tests

consensus/state_processing/src/per_block_processing/errors.rs

@@ -73,6 +73,18 @@ pub enum BlockProcessingError {
         used: u64,
         limit: u64,
     },
+    ExecutionInvalidGasLimitIncrease {
+        limit: u64,
+        parent_limit: u64,
+    },
+    ExecutionInvalidGasLimitDecrease {
+        limit: u64,
+        parent_limit: u64,
+    },
+    ExecutionInvalidGasLimitTooSmall {
+        limit: u64,
+        min: u64,
+    },
     ExecutionInvalidTimestamp {
         expected: u64,
         found: u64,