wsgi_additional: use more execute_batch() in tests

Which removes the last use of Time::now_string().

Also, this is meant to be the last commit around the 'remove
workdir/street-housenumbers-reference-<relation>.lst files' story.

Change-Id: Idb6f2537c2278cacf31d78d251607c5a074400d8

src/context.rs

@@ -89,12 +89,6 @@ pub trait Time {
     /// Calculates the current time.
     fn now(&self) -> time::OffsetDateTime;
 
-    /// The current time as a unix timestamp nanosec string.
-    fn now_string(&self) -> String {
-        let now = self.now();
-        now.unix_timestamp_nanos().to_string()
-    }
-
     /// Delay execution for a given number of seconds.
     fn sleep(&self, seconds: u64);