Updating documentation for adding new unit test files
Removing unneeded sentence from README
Removing uint160_tests.cpp as it DNE
Formatting command line instructions to use ``
fixing 80 char formatting issue in README
fixing more nits
Github-Pull: #8428
Rebased-From: b8db185952c815444b7052092472ef9af3a42e89
As suggested by Greg Maxwell-- unit test to make sure a block
with a double-spend in it doesn't pass validation if half of
the double-spend is already in the memory pool (so full-blown
transaction validation is skipped) when the block is received.
