Correct use of Mobly test_case arg

The --test_case argument of Mobly is meant to be declared once with
multiple values proceeded separated by a space.

Fixed: b/313540348
Change-Id: I2e7cd0df41635b4d9a60649d7445fc96f7940c8b
Reviewed-on: https://fuchsia-review.googlesource.com/c/antlion/+/952458
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Patrick Lu <patricklu@google.com>
Fuchsia-Auto-Submit: Sam Balana <sbalana@google.com>
diff --git a/runner/src/runner.rs b/runner/src/runner.rs
index 12a35eb..986acf1 100644
--- a/runner/src/runner.rs
+++ b/runner/src/runner.rs
@@ -31,9 +31,11 @@
             config.into_os_string().into_string().unwrap(),
         ];
 
-        for test_case in self.test_cases.iter() {
+        if !self.test_cases.is_empty() {
             args.push("--test_case".to_string());
-            args.push(test_case.clone());
+            for test_case in self.test_cases.iter() {
+                args.push(test_case.clone());
+            }
         }
 
         println!(