test_provider.rb: Make a legacy provider test optional.

In some cases such as OpenSSL package in FreeBSD[1], the legacy provider is not
installed intentionally. So, we omit a test depending the legacy provider if the
legacy provider is not loadable.

For the test_openssl_provider_names test, we use base provider[2] instead of
legacy provider, because we would expect the base provider is always loadable
in OpenSSL 3 for now.

* [1] https://www.freshports.org/security/openssl/
* [2] https://wiki.openssl.org/index.php/OpenSSL_3.0#Providers

test/openssl/test_provider.rb

@@ -13,13 +13,13 @@ def test_openssl_provider_name_inspect
 
   def test_openssl_provider_names
     with_openssl <<-'end;'
-      legacy_provider = OpenSSL::Provider.load("legacy")
+      base_provider = OpenSSL::Provider.load("base")
       assert_equal(2, OpenSSL::Provider.provider_names.size)
-      assert_includes(OpenSSL::Provider.provider_names, "legacy")
+      assert_includes(OpenSSL::Provider.provider_names, "base")
 
-      assert_equal(true, legacy_provider.unload)
+      assert_equal(true, base_provider.unload)
       assert_equal(1, OpenSSL::Provider.provider_names.size)
-      assert_not_includes(OpenSSL::Provider.provider_names, "legacy")
+      assert_not_includes(OpenSSL::Provider.provider_names, "base")
     end;
   end
 
@@ -34,7 +34,12 @@ def test_unloaded_openssl_provider
 
   def test_openssl_legacy_provider
     with_openssl(<<-'end;')
-      OpenSSL::Provider.load("legacy")
+      begin
+        OpenSSL::Provider.load("legacy")
+      rescue OpenSSL::Provider::ProviderError
+        omit "Only for OpenSSL with legacy provider"
+      end
+
       algo = "RC4"
       data = "a" * 1000
       key = OpenSSL::Random.random_bytes(16)