This commit moves functions start_node, start_nodes, stop_node and
stop_nodes functions into the BitcoinTestFramework class. It also moves
the bitcoind_processes dict and coverage variables into BitcoinTestFramework.
A few "a->an" and "an->a".
"Shows, if the supplied default SOCKS5 proxy" -> "Shows if the supplied default SOCKS5 proxy". Change made on 3 occurrences.
"without fully understanding the ramification of a command" -> "without fully understanding the ramifications of a command".
Removed duplicate words such as "the the".
Change bumpfee tests to use the spend_one_input function instead of the
create_fund_sign_send function. The latter function would choose transaction
inputs and fees in unpredictable ways depending on the order that tests ran,
which meant that adding new tests could cause old tests to fail, and in general
made bumpfee.py fragile and unpleasant to work with.
