test_fips.rb: Fix the OpenSSL.fips_mode affecting other tests.

Run the test with `assert_separately` for the `false` value of the
`OpenSSL.fips_mode` not to affect other tests.

Author: junaruga

test/openssl/test_fips.rb

@@ -28,8 +28,10 @@ def test_fips_mode_get_is_false_on_fips_mode_disabled
   end
 
   def test_fips_mode_is_reentrant
-    OpenSSL.fips_mode = false
-    OpenSSL.fips_mode = false
+    assert_separately(["-ropenssl"], <<~"end;")
+      OpenSSL.fips_mode = false
+      OpenSSL.fips_mode = false
+    end;
   end
 
   def test_fips_mode_get_with_fips_mode_set