More tests

Robolectric throws a NPE without passing in the ShadowActivity. It appears to be trying to get the real activity instead of the ShadowActivity? Regardless, I'm open to suggestions.

``com.nightscout.android.dexcom.SyncingServiceTest > test_startIntentServiceWithUsbDisconnected FAILED
    java.lang.NullPointerException                            
        at org.robolectric.shadows.ShadowPreferenceManager.getDefaultSharedPreferences(ShadowPreferenceManager.java:21)
        at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java)
        at com.nightscout.android.dexcom.SyncingService.handleActionSync(SyncingService.java:125)
        at com.nightscout.android.dexcom.SyncingServiceTest.test_startIntentServiceWithUsbDisconnected(SyncingServiceTest.java:71)``