mobile/bind: implement iOS benchmarks

Add a XCTestCase based ObjC driver, SeqTest.m, to run the benchmarks
package on iOS.

While we're here, replace "Java" with "Foreign" in test names to
reflect that benchmarks run on both platforms now.

Change-Id: I38a38f3093b4b97961107b5ea66f03cff8e395c3
Reviewed-on: https://go-review.googlesource.com/20259
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/SeqBench.java

@@ -53,7 +53,7 @@ public class SeqBench extends InstrumentationTestCase {
         }
       });
       final Benchmark.I javaRef = new AnI();
-      benchmarks.put("Refjava", new Runnable() {
+      benchmarks.put("Refforeign", new Runnable() {
         @Override public void run() {
           Benchmark.Ref(javaRef);
         }