cmd/devp2p: refactor eth test suite (ethereum#22843)

This PR refactors the eth test suite to make it more readable and
easier to use. Some notable differences:

- A new file helpers.go stores all of the methods used between
  both eth66 and eth65 and below tests, as well as methods shared
  among many test functions.
- suite.go now contains all of the test functions for both eth65
  tests and eth66 tests.
- The utesting.T object doesn't get passed through to other helper methods,
  but is instead only used within the scope of the test function,
  whereas helper methods return errors, so only the test function
  itself can fatal out in the case of an error.
- The full test suite now only takes 13.5 seconds to run.

cmd/devp2p/internal/ethtest/chain.go

@@ -54,10 +54,24 @@ func (c *Chain) Len() int {
 	return len(c.blocks)
 }
 
-// TD calculates the total difficulty of the chain.
-func (c *Chain) TD(height int) *big.Int { // TODO later on channge scheme so that the height is included in range
+// TD calculates the total difficulty of the chain at the
+// chain head.
+func (c *Chain) TD() *big.Int {
 	sum := big.NewInt(0)
-	for _, block := range c.blocks[:height] {
+	for _, block := range c.blocks[:c.Len()] {
+		sum.Add(sum, block.Difficulty())
+	}
+	return sum
+}
+
+// TotalDifficultyAt calculates the total difficulty of the chain
+// at the given block height.
+func (c *Chain) TotalDifficultyAt(height int) *big.Int {
+	sum := big.NewInt(0)
+	if height >= c.Len() {
+		return sum
+	}
+	for _, block := range c.blocks[:height+1] {
 		sum.Add(sum, block.Difficulty())
 	}
 	return sum