Add a new self test to entropy module

The self test is a quick way to check at startup whether the entropy
sources are functioning correctly. The self test only polls 8 bytes
from the default entropy source and performs the following checks:

- The bytes are not all 0x00 or 0xFF.
- The hardware does not return an error when polled.
- The entropy does not provide data in a patter. Only check pattern
  at byte, word and long word sizes.

tests/suites/test_suite_entropy.function

@@ -381,5 +381,8 @@ void entropy_nv_seed( char *read_seed_str )
 void entropy_selftest( )
 {
     TEST_ASSERT( mbedtls_entropy_self_test( 0 ) == 0 );
+#if !defined(MBEDTLS_TEST_NULL_ENTROPY) && defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
+    TEST_ASSERT( mbedtls_entropy_self_test_bias( 0 ) == 0 );
+#endif
 }
 /* END_CASE */