fix: testnet datadir could not be created first

[SomberNight]

see #3111

Electrum could only start in testnet mode if it had already started in mainnet mode before (so that it created the parent folder of the datadir already).

$ ./electrum -v --testnet
Traceback (most recent call last):
  File "./electrum", line 360, in <module>
    config = SimpleConfig(config_options)
  File "/home/user/wspace/electrum/lib/simple_config.py", line 74, in __init__
    self.path = self.electrum_path()
  File "/home/user/wspace/electrum/lib/simple_config.py", line 95, in electrum_path
    os.mkdir(path)
FileNotFoundError: [Errno 2] No such file or directory: '/home/user/.electrum/testnet'

$ ./electrum -v --testnet --dir=tmp
Traceback (most recent call last):
  File "./electrum", line 360, in <module>
    config = SimpleConfig(config_options)
  File "/home/user/wspace/electrum/lib/simple_config.py", line 74, in __init__
    self.path = self.electrum_path()
  File "/home/user/wspace/electrum/lib/simple_config.py", line 95, in electrum_path
    os.mkdir(path)
FileNotFoundError: [Errno 2] No such file or directory: 'tmp/testnet'

[coveralls]

Coverage increased (+0.007%) to 42.76% when pulling 650f504 on SomberNight:make_dir_testnet into b786249 on spesmilo:master.