xpi: update missing ext key usage code signing unit test cert

generated with:

openssl req -x509 -key test_no_eku.key -keyform PEM -out cert.pem -days 36500 -addext keyUsage=keyCertSign

signer/xpi/xpi_test.go

@@ -1366,26 +1366,27 @@ a05oGDnmq7R+
 		Type: Type,
 		Mode: ModeAddOn,
 		Certificate: `-----BEGIN CERTIFICATE-----
-MIIDjTCCAnWgAwIBAgIJAPzn72+kKJ31MA0GCSqGSIb3DQEBCwUAMHUxCzAJBgNV
-BAYTAlVTMQswCQYDVQQIDAJDQTEWMBQGA1UEBwwNTW91bnRhaW4gVmlldzEXMBUG
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-zQ==
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 -----END CERTIFICATE-----
 `,
 		PrivateKey: `-----BEGIN PRIVATE KEY-----