qa: Windows fixups for functional tests #12917

Just two minor fixups to have less errors when the tests run on native windows.

• Strip whitespace from lines when reading from a notification file
• Instead of clumsily creating a file with weird permissions, just create a folder for the same effect in mempool_persist.py

The failure for removing the file with weird permissions: PermissionError: [WinError 5] Access is denied: 'C:\\Users\\b\\AppData\\Local\\Temp\\1\\bitcoin_test_runner_20180318_140545\\mempool_persist_445\\node1\\regtest\\mempool.dat.new' and the failure in notifications:

AssertionError: not([
'0de8e8bb3d20037e2d8590a7da3016592f42d857aae4db6cf4ec19ad168ae6bb', 
'392fd1853324a63fe4a3f84c2dafbb204f608d91f48c7b093fb459cc7ec60fe3', 
'39b07286b19d08e0c14528289663864a466dd53b921d08845c99f8aa6bbe6aa7', 
'45e541f38d93565297f4a1c1538e4623efdec4cf73d4d08884ca0d5d29b773cb', 
'47216f1684c46d870d98890bc258ea1e46b10565e093cc77d760f8ffb0928093', 
'47b7fb4d72de6027173ccc2cb04c7d8432a5f6afc1ee3deecf82d7873724fac8', 
'4c11025fa0abe62585272aebf08f814ed316d876da6362b2e0964d9431c108ee', 
'55006dd51e55b96c5eff2e953460a90c43064082020127d7e2eb8cb9de021ac9', 
'5db418f4eaadfaddde2de6a4f2a4e8b07a1b2b358eae70c320f5ef5050cecc0e', 
'7012e96c86de31e8c386ad2f88e2f58bd633c43adba4fbaa010acdbee4470401'] == 
['0de8e8bb3d20037e2d8590a7da3016592f42d857aae4db6cf4ec19ad168ae6bb ', 
'392fd1853324a63fe4a3f84c2dafbb204f608d91f48c7b093fb459cc7ec60fe3 ', 
'39b07286b19d08e0c14528289663864a466dd53b921d08845c99f8aa6bbe6aa7 ', 
'45e541f38d93565297f4a1c1538e4623efdec4cf73d4d08884ca0d5d29b773cb ', 
'47216f1684c46d870d98890bc258ea1e46b10565e093cc77d760f8ffb0928093 ', 
'47b7fb4d72de6027173ccc2cb04c7d8432a5f6afc1ee3deecf82d7873724fac8 ', 
'4c11025fa0abe62585272aebf08f814ed316d876da6362b2e0964d9431c108ee ', 
'55006dd51e55b96c5eff2e953460a90c43064082020127d7e2eb8cb9de021ac9 ', 
'5db418f4eaadfaddde2de6a4f2a4e8b07a1b2b358eae70c320f5ef5050cecc0e ', 
'7012e96c86de31e8c386ad2f88e2f58bd633c43adba4fbaa010acdbee4470401 '])

utACK https://github.com/bitcoin/bitcoin/pull/12917/commits/fab9095d4071159f88eeefa5c1a750960fd2fbb9

I suspect the extra whitespace on windows is the carriage return \r they add before newlines.