Add a reference configuration without any asymmetric cryptography

Add a reference configuration with most symmetric cryptographic
algorithms enabled, but without any asymmetric cryptography. This
checks that we don't have spurious unexpected dependencies on
asymmetric-only modules such as bignum.

Keep HAVE_ASM disabled because it's platform-specific.

Keep HAVEGE disabled because it's untested and not recommended.

Keep MEMORY_BUFFER_ALLOC out because it isn't related to cryptography
at all and it makes memory sanitizers ineffective.

Keep THREADING disabled because it requires special build options.

tests/scripts/test-ref-configs.pl

@@ -17,6 +17,8 @@ use warnings;
 use strict;
 
 my %configs = (
+    'config-symmetric-only.h' => {
+    },
     'config-suite-b.h' => {
     },
 );