clearOldFiles() test must run on the emulator

It seems that Robolectric does not implement android.system.Os so the
atime checking code cannot be tested there.  Works fine on the emulator.

app/src/androidTest/java/org/fdroid/fdroid/CleanCacheServiceTest.java

@@ -0,0 +1,61 @@
+package org.fdroid.fdroid;
+
+import android.support.test.runner.AndroidJUnit4;
+
+import org.apache.commons.io.FileUtils;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import java.io.File;
+import java.io.IOException;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+@RunWith(AndroidJUnit4.class)
+public class CleanCacheServiceTest {
+
+    public static final String TAG = "CleanCacheServiceTest";
+
+    @Test
+    public void testClearOldFiles() throws IOException, InterruptedException {
+        File tempDir = new File(System.getProperty("java.io.tmpdir"));
+        assertTrue(tempDir.isDirectory());
+        assertTrue(tempDir.canWrite());
+
+        File dir = new File(tempDir, "F-Droid-test.clearOldFiles");
+        FileUtils.deleteQuietly(dir);
+        assertTrue(dir.mkdirs());
+        assertTrue(dir.isDirectory());
+
+        File first = new File(dir, "first");
+        first.deleteOnExit();
+
+        File second = new File(dir, "second");
+        second.deleteOnExit();
+
+        assertFalse(first.exists());
+        assertFalse(second.exists());
+
+        assertTrue(first.createNewFile());
+        assertTrue(first.exists());
+
+        Thread.sleep(7000);
+        assertTrue(second.createNewFile());
+        assertTrue(second.exists());
+
+        CleanCacheService.clearOldFiles(dir, 3000);  // check all in dir
+        assertFalse(first.exists());
+        assertTrue(second.exists());
+
+        Thread.sleep(7000);
+        CleanCacheService.clearOldFiles(second, 3000);  // check just second file
+        assertFalse(first.exists());
+        assertFalse(second.exists());
+
+        // make sure it doesn't freak out on a non-existant file
+        File nonexistant = new File(tempDir, "nonexistant");
+        CleanCacheService.clearOldFiles(nonexistant, 1);
+        CleanCacheService.clearOldFiles(null, 1);
+    }
+}