Add test for CA forgery attempt

As we accept EE certs that are explicitly trusted (in the list of trusted
roots) and usually look for parent by subject, and in the future we might want
to avoid checking the self-signature on trusted certs, there could a risk that we
incorrectly accept a cert that looks like a trusted root except it doesn't
have the same key. This test ensures this will never happen.

tests/data_files/Makefile

@@ -97,6 +97,13 @@ server5-ss-expired.crt: server5.key
 	$(FAKETIME) -f -3653d $(OPENSSL) req -x509 -new -subj "/C=UK/O=mbed TLS/OU=testsuite/CN=localhost" -days 3653 -sha256 -key $< -out $@
 all_final += server5-ss-expired.crt
 
+# try to forge a copy of test-int-ca3 with different key
+server5-ss-forgeca.crt: server5.key
+	$(FAKETIME) '2015-09-01 14:08:43' $(OPENSSL) req -x509 -new -subj "/C=UK/O=mbed TLS/CN=mbed TLS Test intermediate CA 3" -set_serial 77 -config $(test_ca_config_file) -extensions noext_ca -days 3650 -sha256 -key $< -out $@
+all_final += server5-ss-forgeca.crt
+
+
+
 
 ################################################################
 #### Meta targets

tests/data_files/server5-ss-forgeca.crt

@@ -0,0 +1,11 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----

tests/data_files/test-ca.opensslconf

@@ -11,3 +11,6 @@ commonName = PolarSSL Test CA
 subjectKeyIdentifier=hash
 authorityKeyIdentifier=keyid:always,issuer:always
 basicConstraints = CA:true
+
+[noext_ca]
+basicConstraints = CA:true