FullBlockTestGenerator, BitcoindComparisonTool, FullPrunedBlockChainTest cleanup/modernization - #4350

…blic

…tate

For readability, parse the port argument right after the runExpensiveTests
argument.

…local var

Prior to this commit in each class, the FullBlockTestGenerator is constructed and
used for a single getBlocksToTest() leaving the local variable `generator`
unused after that. To make the code more readable we simply call getBlocksToTest()
on the instance and save the resulting RuleList in a local variable.