Correct srp test

There actually is a "srp" feature to check the availability on
Reviewed-by: NStephen Henson <steve@openssl.org>

test/recipes/90-test_srp.t

@@ -2,4 +2,4 @@
 
 use OpenSSL::Test::Simple;
 
-simple_test("test_srp", "srptest");
+simple_test("test_srp", "srptest", "srp");