tests: make init apksigner test use existing test keystore
tests: make init apksigner test use existing test keystore

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'

```

f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver