misc/ios: retry iOS launch even if no device app path was found

Now that the iOS exec wrapper uninstalls any existing test app before
installing a new, looking up the device app path might fail. Before,
the lookup always succeeded (even though the path reported might be
stale).

For the iOS builder.

Change-Id: I5667b6fae15f88745bdee796db219a429a26e203
Reviewed-on: https://go-review.googlesource.com/112075
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

misc/ios/go_darwin_arm_exec.go

@@ -469,12 +469,17 @@ func run(appdir, bundleID string, args []string) error {
 	}
 	attempt := 0
 	for {
-		// The device app path is constant for a given installed app,
-		// but the device might not return a stale device path for
-		// a newly overwritten app, so retry the lookup as well.
+		// The device app path reported by the device might be stale, so retry
+		// the lookup of the device path along with the lldb launching below.
 		deviceapp, err := findDeviceAppPath(bundleID)
 		if err != nil {
-			return err
+			// The device app path might not yet exist for a newly installed app.
+			if attempt == 5 {
+				return err
+			}
+			attempt++
+			time.Sleep(5 * time.Second)
+			continue
 		}
 		lldb := exec.Command(
 			"python",