Implement sleepMillis in MockTimeService to fix main build failures

Signed-off-by: Eamonn Mansour <47121388+eamansour@users.noreply.github.com>

galasa-extensions-parent/dev.galasa.extensions.mocks/src/main/java/dev/galasa/extensions/mocks/MockTimeService.java

@@ -25,4 +25,10 @@ public Instant now() {
     public void setCurrentTime(Instant currentTime) {
         this.currentTime = currentTime;
     }
+
+    @Override
+    public void sleepMillis(long millisToSleep) throws InterruptedException {
+        // Pretend we are sleeping, so the current time advances.
+        setCurrentTime(currentTime.plusMillis(millisToSleep));
+    }
 }