tests: skip superfluous cert generation in EnsureCertFor()

The certificate generated by `PkiUtility::NewCert()` is self-signed,
and so the subsequent `PkiUtility::SignCsr()` call is required.
However, `PkiUtility::SignCsr()` doesn't reuse existin cert, instead
it'll generate a fresh one on its own. So, skip the first one entirely!

Author: yhabteab

test/remote-certificate-fixture.hpp

@@ -53,7 +53,7 @@ struct CertificateFixture : ConfigurationDataDirFixture
 		cert.csrFile = (m_CertsDir / (name + ".csr")).string();
 
 		if (overrideExisting || !Utility::PathExists(cert.crtFile)) {
-			PkiUtility::NewCert(name, cert.keyFile, cert.csrFile, cert.crtFile);
+			PkiUtility::NewCert(name, cert.keyFile, cert.csrFile, "");
 			PkiUtility::SignCsr(cert.csrFile, cert.crtFile);
 		}