runner: Don't generate an RSA key on startup.

RSA keygen isn't the fastest. Just use the existing one in
rsaCertificate.

Change-Id: Icd151232928e67e0a7d5becabf9dc96b0e9bfa22
Reviewed-on: https://boringssl-review.googlesource.com/c/34764
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>

ssl/test/runner/runner.go

@@ -14153,7 +14153,7 @@ func addECDSAKeyUsageTests() {
 }
 
 func addRSAKeyUsageTests() {
-	priv, err := rsa.GenerateKey(rand.Reader, 2048)
+	priv := rsaCertificate.PrivateKey.(*rsa.PrivateKey)
 
 	serialNumberLimit := new(big.Int).Lsh(big.NewInt(1), 128)
 	serialNumber, err := rand.Int(rand.Reader, serialNumberLimit)