This test does not need to generate a keystore, so using the existing one
reduces the run time from 4 seconds to 0.040 seconds. Also, if makes this
mystery error on macOS go away:
```
Traceback (most recent call last):
File "/Users/travis/build/fdroidtravis/fdroidserver/tests/init.TestCase", line 66, in test_main_in_empty_dir
fdroidserver.init.main()
File "/Users/travis/build/fdroidtravis/fdroidserver/fdroidserver/init.py", line 231, in main
common.genkeystore(c)
File "/Users/travis/build/fdroidtravis/fdroidserver/fdroidserver/common.py", line 3434, in genkeystore
cmd = [config['keytool'], '-genkey',
KeyError: 'keytool'
```
