Fix new clang -Wthread-safety-analysis warnings

Returning a reference to a GUARDED_BY variable while only holding the lock in the getter lets callers access the variable without holding the lock.

See llvm/llvm-project#67776.

This is only used for testing, so just return a copy for simplicity.

Author: aeubanks

src/core/lib/security/security_connector/tls/tls_security_connector.h

@@ -205,12 +205,12 @@ class TlsServerSecurityConnector final : public grpc_server_security_connector {
     return server_handshaker_factory_;
   };
 
-  const absl::optional<absl::string_view>& RootCertsForTesting() {
+  const absl::optional<absl::string_view> RootCertsForTesting() {
     MutexLock lock(&mu_);
     return pem_root_certs_;
   }
 
-  const absl::optional<PemKeyCertPairList>& KeyCertPairListForTesting() {
+  const absl::optional<PemKeyCertPairList> KeyCertPairListForTesting() {
     MutexLock lock(&mu_);
     return pem_key_cert_pair_list_;
   }