Add |RTCUIApplicationStatusObserver sharedInstance| call in ios test AppDelegate.

We want to perform the observation setup as soon as possible to avoid deadlocking,
especially for test scenario where most of the work is done on main thread.

BUG=webrtc:6634

Review-Url: https://codereview.webrtc.org/3012913002
Cr-Commit-Position: refs/heads/master@{#19771}

webrtc/test/BUILD.gn

@@ -119,6 +119,9 @@ if (is_ios) {
       "ios/test_support.h",
       "ios/test_support.mm",
     ]
+    deps = [
+      "../sdk:common_objc",
+    ]
   }
 }
 

webrtc/test/ios/test_support.mm

@@ -12,6 +12,8 @@
 
 #include "webrtc/test/ios/test_support.h"
 
+#import "webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.h"
+
 // Springboard will kill any iOS app that fails to check in after launch within
 // a given time. Starting a UIApplication before invoking TestSuite::Run
 // prevents this from happening.
@@ -60,6 +62,11 @@ static char **g_argv;
   // root view controller. Set an empty one here.
   [_window setRootViewController:[[UIViewController alloc] init]];
 
+  // We want to call `RTCUIApplicationStatusObserver sharedInstance` as early as
+  // possible in the application lifecycle to set observation properly.
+  __unused RTCUIApplicationStatusObserver *observer =
+      [RTCUIApplicationStatusObserver sharedInstance];
+
   // Queue up the test run.
   [self performSelector:@selector(runTests) withObject:nil afterDelay:0.1];
   return YES;