[ruby/openssl] CI: Add OpenSSL FIPS mode case.

test/openssl/fixtures/ssl/openssl_fips.cnf.tmpl:

I referred to the following document for the openssl config file for FIPS mode.
<https://www.openssl.org/docs/manmaster/man7/fips_module.html>
- Making all applications use the FIPS module by default

It seems that the `.include` syntax only requires the absolute path.
So, the placeholder OPENSSL_DIR in the template file is replaced with the
actual OpenSSL directory.

.github/workflows/test.yml:

The `TEST_RUBY_OPENSSL_FIPS_ENABLED` environment variable is set
in the FIPS mode CI case. It can be used in the unit tests.

https://github.com/ruby/openssl/commit/18b017218c

test/openssl/fixtures/ssl/openssl_fips.cnf.tmpl

@@ -0,0 +1,19 @@
+config_diagnostics = 1
+openssl_conf = openssl_init
+
+# It seems that the .include needs an absolute path.
+.include OPENSSL_DIR/ssl/fipsmodule.cnf
+
+[openssl_init]
+providers = provider_sect
+alg_section = algorithm_sect
+
+[provider_sect]
+fips = fips_sect
+base = base_sect
+
+[base_sect]
+activate = 1
+
+[algorithm_sect]
+default_properties = fips=yes