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/random.h

@@ -58,4 +58,9 @@ static const ssize_t NUM_OS_RANDOM_BYTES = 32;
  */
 void GetOSRand(unsigned char *ent32);
 
+/** Check that OS randomness is available and returning the requested number
+ * of bytes.
+ */
+bool Random_SanityCheck();
+
 #endif // BITCOIN_RANDOM_H