Add a signature count to MTRTestKeys. (#28452)

This lets us check whether CASE resumption is happening.

Author: bzbarsky-apple

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

@@ -25,6 +25,9 @@ NS_ASSUME_NONNULL_BEGIN
 
 @property (readonly, nonatomic) NSData * publicKeyData;
 
+// Count of how many times this keypair has been used to signMessageECDSA_DER.
+@property (readonly, nonatomic, assign) unsigned signatureCount;
+
 - (instancetype)init;
 
 @end

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

@@ -69,11 +69,15 @@ - (instancetype)init
     }
     _publicKey = SecKeyCopyPublicKey(_privateKey);
 
+    _signatureCount = 0;
+
     return self;
 }
 
 - (NSData *)signMessageECDSA_DER:(NSData *)message
 {
+    ++_signatureCount;
+
     CFErrorRef error = NULL;
     CFDataRef outData
         = SecKeyCreateSignature(_privateKey, kSecKeyAlgorithmECDSASignatureMessageX962SHA256, (__bridge CFDataRef) message, &error);