misc: Add comment for failing tests in FIPS environments (#12188)

Summary:
In FIPS enabled environments OpenSSL is restricting access to certain APIs due to them being insecure. Velox supports functions that calculate the results using these obsolete functions. The tests will fail if run.

Pull Request resolved: #12188

Reviewed By: kgpai

Differential Revision: D68746609

Pulled By: xiaoxmeng

fbshipit-source-id: c428ea8cd7170436f784d9dbab96282150983f1f

velox/functions/prestosql/tests/BinaryFunctionsTest.cpp

@@ -257,6 +257,8 @@ TEST_F(BinaryFunctionsTest, HmacSha512) {
   EXPECT_EQ(std::nullopt, hmacSha512(std::nullopt, "velox"));
 }
 
+// Note: this test fails in a FIPS enabled environment because OpenSSL restricts
+// usage of MD5 for hmacs.
 TEST_F(BinaryFunctionsTest, HmacMd5) {
   const auto hmacMd5 = [&](std::optional<std::string> arg,
                            std::optional<std::string> key) {