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'

```

tests/init.TestCase

@@ -6,6 +6,7 @@ import inspect
 import logging
 import os
 import optparse
+import shutil
 import sys
 import tempfile
 import unittest
@@ -52,9 +53,12 @@ class InitTest(unittest.TestCase):
         self.assertIsNone(config.get('keypass'))
 
     def test_main_in_empty_dir(self):
+        """Test that `fdroid init` will find apksigner and add it to the config"""
         testdir = tempfile.mkdtemp(prefix=inspect.currentframe().f_code.co_name, dir=self.tmpdir)
         os.chdir(testdir)
 
+        shutil.copy(os.path.join(self.basedir, 'keystore.jks'), testdir)
+
         bindir = os.path.join(os.getcwd(), 'bin')
         os.mkdir(bindir)
         apksigner = os.path.join(bindir, 'apksigner')
@@ -62,7 +66,7 @@ class InitTest(unittest.TestCase):
         os.chmod(apksigner, 0o755)
         os.environ['PATH'] = bindir
 
-        sys.argv = ['fdroid init']
+        sys.argv = ['fdroid init', '--keystore', 'keystore.jks', '--repo-keyalias=sova']
         fdroidserver.init.main()
         with open('config.yml') as fp:
             config = yaml.safe_load(fp)