test/openssl/test_ossl.rb: use clock_gettime for measuring time

The benchmark library is planned to become a bundled gem in Ruby 3.5.
While we can add it in our Gemfile, it is only used in
test_memcmp_timing and the usage can be easily replaced with a few
Process.clock_gettime calls.

Author: rhenium

test/openssl/test_ossl.rb

@@ -42,12 +42,6 @@ def test_secure_compare
   end
 
   def test_memcmp_timing
-    begin
-      require "benchmark"
-    rescue LoadError
-      pend "Benchmark is not available in this environment. Please install it with `gem install benchmark`."
-    end
-
     # Ensure using fixed_length_secure_compare takes almost exactly the same amount of time to compare two different strings.
     # Regular string comparison will short-circuit on the first non-matching character, failing this test.
     # NOTE: this test may be susceptible to noise if the system running the tests is otherwise under load.
@@ -58,8 +52,14 @@ def test_memcmp_timing
 
     a_b_time = a_c_time = 0
     100.times do
-      a_b_time += Benchmark.measure { 100.times { OpenSSL.fixed_length_secure_compare(a, b) } }.real
-      a_c_time += Benchmark.measure { 100.times { OpenSSL.fixed_length_secure_compare(a, c) } }.real
+      t1 = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+      100.times { OpenSSL.fixed_length_secure_compare(a, b) }
+      t2 = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+      100.times { OpenSSL.fixed_length_secure_compare(a, c) }
+      t3 = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+
+      a_b_time += t2 - t1
+      a_c_time += t3 - t2
     end
     assert_operator(a_b_time, :<, a_c_time * 10, "fixed_length_secure_compare timing test failed")
     assert_operator(a_c_time, :<, a_b_time * 10, "fixed_length_secure_compare timing test failed")