implement copyPublicKey for MTRTestKeys; add TODO note about opti…

…onal method calls

src/darwin/Framework/CHIPTests/MTRCertificateTests.m

@@ -126,6 +126,9 @@ - (void)testGenerateIntermediateCert
 
     __auto_type * intermediateKeys = [[MTRTestKeys alloc] init];
     XCTAssertNotNil(intermediateKeys);
+    // TODO:  there are several places in our tests where we call `copyPublicKey` on objects without first
+    // seeing if it's implemented by the receiver.  (the `MTRKeypair` protocol says both `publicKey` and
+    // `copyPublicKey` are optional.)
     __auto_type * intermediatePublicKey = intermediateKeys.copyPublicKey;
     XCTAssert(intermediatePublicKey != NULL);
     CFAutorelease(intermediatePublicKey);

src/darwin/Framework/CHIPTests/TestHelpers/MTRTestKeys.m

@@ -31,6 +31,13 @@ - (NSData *)publicKeyData
     return (__bridge_transfer NSData *) SecKeyCopyExternalRepresentation([self publicKey], nil);
 }
 
+- (SecKeyRef)copyPublicKey {
+    // because this varies significantly from the actual implementations,
+    // it's probably not a great idea to rely on this for memory leak behavior information.
+    CFRetain(_publicKey);
+    return _publicKey;
+}
+
 - (instancetype)init
 {
     if (!(self = [super init])) {