Fix the false value of the OpenSSL.fips_mode affecting other tests.

We need to reset the `false` value of the `OpenSSL.fips_mode` not to affect
other tests.

Author: junaruga

test/openssl/test_fips.rb

@@ -28,8 +28,12 @@ def test_fips_mode_get_is_false_on_fips_mode_disabled
   end
 
   def test_fips_mode_is_reentrant
+    original_fips_mode = OpenSSL.fips_mode
+
     OpenSSL.fips_mode = false
     OpenSSL.fips_mode = false
+  ensure
+    OpenSSL.fips_mode = original_fips_mode
   end
 
   def test_fips_mode_get_with_fips_mode_set