test/openssl/test_digest: do not test constants for legacy algorithms

Remove availability test for MD4 and RIPEMD160 as they are considered
legacy and may be missing depending on the compile-time options of
OpenSSL. OpenSSL 3.0 by default disables them.

test/openssl/test_digest.rb

@@ -54,7 +54,7 @@ def test_reset
   end
 
   def test_digest_constants
-    %w{MD4 MD5 RIPEMD160 SHA1 SHA224 SHA256 SHA384 SHA512}.each do |name|
+    %w{MD5 SHA1 SHA224 SHA256 SHA384 SHA512}.each do |name|
       assert_not_nil(OpenSSL::Digest.new(name))
       klass = OpenSSL::Digest.const_get(name.tr('-', '_'))
       assert_not_nil(klass.new)