sanity: Move OS random to sanity check function

Move the OS random test to a sanity check function that is called every
time bitcoind is initialized.

Keep `src/test/random_tests.cpp` for the case that later random tests
are added, and keep a rudimentary test that just calls the sanity check.

src/init.cpp

@@ -687,9 +687,15 @@ bool InitSanityCheck(void)
         InitError("Elliptic curve cryptography sanity check failure. Aborting.");
         return false;
     }
+
     if (!glibc_sanity_test() || !glibcxx_sanity_test())
         return false;
 
+    if (!Random_SanityCheck()) {
+        InitError("OS cryptographic RNG sanity check failure. Aborting.");
+        return false;
+    }
+
     return true;
 }