Set path to databases.properties through system property.

This makes it possible to easily run tests on multiple testing VMs with
different database configurations from the same checkout of Slick.

Review by @cvogt

slick-testkit/src/main/scala/com/typesafe/slick/testkit/util/TestDB.scala

@@ -28,7 +28,7 @@ object TestDB {
   }
   private lazy val dbProps = {
     val p = new Properties
-    val f = new File("test-dbs", "databases.properties")
+    val f = new File(sys.props.getOrElse("slick.testkit.dbprops", "test-dbs/databases.properties"))
     if(f.isFile) {
       val in = new FileInputStream(f)
       try { p.load(in) } finally { in.close() }
@@ -208,4 +208,4 @@ object ExternalTestDB {
       new URLClassLoader(Array(new URL(url)), getClass.getClassLoader).loadClass(driverClass).newInstance.asInstanceOf[Driver]
     )
   }
-}
+}