Add unittest for default severity level of RTCCallbackLogger

(I forgot to include this change in https://webrtc-review.googlesource.com/c/src/+/87800)


Bug: webrtc:9509
Change-Id: I1f4a81e6b235ccde75b9942e2a77b2d6d0fe1364
Reviewed-on: https://webrtc-review.googlesource.com/88000
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23915}

sdk/objc/Framework/UnitTests/RTCCallbackLogger_xctest.m

@@ -30,6 +30,10 @@
   self.logger = nil;
 }
 
+- (void)testDefaultSeverityLevel {
+  XCTAssertEqual(self.logger.severity, RTCLoggingSeverityInfo);
+}
+
 - (void)testCallbackGetsCalledForAppropriateLevel {
   self.logger.severity = RTCLoggingSeverityWarning;