DEV: test_concurrent_access causing segfaults

In Ruby <=2.7, SecureRandom.hex lazily defined `gen_random'.
While the method definition itself was protected by a mutex,
another thread calling `gen_random' did not take the mutex so
it would attempt to call the method while another method was
still defining it.

Ruby 3.0+ has commit 475c8701d74ebebe (Make SecureRandom support
Ractor, 2020-09-04) which also made it thread-safe in addition
to being Ractor-safe.

test/mini_racer_test.rb

@@ -583,6 +583,10 @@ raise FooError, "I like foos"
 
     equals_after_sleep = {}
 
+    # workaround Rubies prior to commit 475c8701d74ebebe
+    # (Make SecureRandom support Ractor, 2020-09-04)
+    SecureRandom.hex
+
     (1..10).map do |i|
       Thread.new {
         random = SecureRandom.hex