clean up config loading in light client tests (#6574)

The consensus-spec-tests routinely add random experimental features
to their config.yaml which can be ignored.

tests/consensus_spec/test_fixture_light_client_data_collection.nim

@@ -130,10 +130,8 @@ proc loadSteps(
 proc runTest(suiteName, path: string, consensusFork: static ConsensusFork) =
   let relativePathComponent = path.relativeTestPathComponent()
   test "Light client - Data collection - " & relativePathComponent:
-    let (cfg, unknowns) = readRuntimeConfig(path/"config.yaml")
-    doAssert unknowns.len == 0
-
     let
+      (cfg, _) = readRuntimeConfig(path/"config.yaml")
       initial_state = loadForkedState(
         path/"initial_state.ssz_snappy", consensusFork)
       db = BeaconChainDB.new("", cfg = cfg, inMemory = true)

tests/consensus_spec/test_fixture_light_client_sync.nim

@@ -134,11 +134,6 @@ proc runTest(suiteName, path: string) =
         Exception, IOError, PresetFileError, PresetIncompatibleError].} =
       let (cfg, _) = readRuntimeConfig(path/"config.yaml")
 
-      when false:
-        # TODO evaluate whether this is useful and if so, fix it
-        # Unhandled defect: nimbus-eth2/tests/consensus_spec/test_fixture_light_client_sync.nim(131, 16) `unknowns.len == 0` Unknown config constants: @["MAXIMUM_GOSSIP_CLOCK_DISPARITY", "ATTESTATION_PROPAGATION_SLOT_RANGE", "MAX_REQUEST_BLOCKS", "SUBNETS_PER_NODE", "TTFB_TIMEOUT", "MIN_EPOCHS_FOR_BLOCK_REQUESTS", "MESSAGE_DOMAIN_VALID_SNAPPY", "ATTESTATION_SUBNET_EXTRA_BITS", "MAX_CHUNK_SIZE", "EPOCHS_PER_SUBNET_SUBSCRIPTION", "GOSSIP_MAX_SIZE", "ATTESTATION_SUBNET_PREFIX_BITS", "MESSAGE_DOMAIN_INVALID_SNAPPY", "RESP_TIMEOUT"] [AssertionDefect]
-        doAssert unknowns.len == 0, "Unknown config constants: " & $unknowns
-
       type TestMetaYaml {.sparse.} = object
         genesis_validators_root: string
         trusted_block_root: string