Make the fallback behavior of mbedtls_test_rnd_buffer_rand optional

If a fallback is not explicitly configured in the
mbedtls_test_rnd_buf_info structure, fail after the buffer is
exhausted.

There is no intended behavior change in this commit: all existing uses
of mbedtls_test_rnd_buffer_rand() have been updated to set
mbedtls_test_rnd_std_rand as the fallback.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_ecdh.function b/tests/suites/test_suite_ecdh.function
index 3ab96fa..da0531b 100644
--- a/tests/suites/test_suite_ecdh.function
+++ b/tests/suites/test_suite_ecdh.function
@@ -240,6 +240,8 @@
 
     rnd_info_A.buf = rnd_buf_A->x;
     rnd_info_A.length = rnd_buf_A->len;
+    rnd_info_A.fallback_f_rng = mbedtls_test_rnd_std_rand;
+    rnd_info_A.fallback_p_rng = NULL;
 
     /* Fix rnd_buf_A->x by shifting it left if necessary */
     if( grp.nbits % 8 != 0 )
@@ -256,6 +258,8 @@
 
     rnd_info_B.buf = rnd_buf_B->x;
     rnd_info_B.length = rnd_buf_B->len;
+    rnd_info_B.fallback_f_rng = mbedtls_test_rnd_std_rand;
+    rnd_info_B.fallback_p_rng = NULL;
 
     /* Fix rnd_buf_B->x by shifting it left if necessary */
     if( grp.nbits % 8 != 0 )
@@ -362,9 +366,13 @@
     mbedtls_ecdh_init( &srv );
     mbedtls_ecdh_init( &cli );
 
+    rnd_info_A.fallback_f_rng = mbedtls_test_rnd_std_rand;
+    rnd_info_A.fallback_p_rng = NULL;
     rnd_info_A.buf = dA->x;
     rnd_info_A.length = dA->len;
 
+    rnd_info_B.fallback_f_rng = mbedtls_test_rnd_std_rand;
+    rnd_info_B.fallback_p_rng = NULL;
     rnd_info_B.buf = dB->x;
     rnd_info_B.length = dB->len;
 
diff --git a/tests/suites/test_suite_ecdsa.function b/tests/suites/test_suite_ecdsa.function
index 58cedc1..2cb8925 100644
--- a/tests/suites/test_suite_ecdsa.function
+++ b/tests/suites/test_suite_ecdsa.function
@@ -292,6 +292,8 @@
     TEST_ASSERT( mbedtls_mpi_read_string( &d, 16, d_str ) == 0 );
     TEST_ASSERT( mbedtls_mpi_read_string( &r_check, 16, r_str ) == 0 );
     TEST_ASSERT( mbedtls_mpi_read_string( &s_check, 16, s_str ) == 0 );
+    rnd_info.fallback_f_rng = mbedtls_test_rnd_std_rand;
+    rnd_info.fallback_p_rng = NULL;
     rnd_info.buf = rnd_buf->x;
     rnd_info.length = rnd_buf->len;
 
diff --git a/tests/suites/test_suite_pkcs1_v15.function b/tests/suites/test_suite_pkcs1_v15.function
index d1c0fc1..a7fb2a5 100644
--- a/tests/suites/test_suite_pkcs1_v15.function
+++ b/tests/suites/test_suite_pkcs1_v15.function
@@ -19,6 +19,8 @@
     mbedtls_test_rnd_buf_info info;
     mbedtls_mpi N, E;
 
+    info.fallback_f_rng = mbedtls_test_rnd_std_rand;
+    info.fallback_p_rng = NULL;
     info.buf = rnd_buf->x;
     info.length = rnd_buf->len;
 
@@ -268,6 +270,8 @@
     mbedtls_mpi N, P, Q, E;
     mbedtls_test_rnd_buf_info info;
 
+    info.fallback_f_rng = mbedtls_test_rnd_std_rand;
+    info.fallback_p_rng = NULL;
     info.buf = rnd_buf->x;
     info.length = rnd_buf->len;
 
diff --git a/tests/suites/test_suite_pkcs1_v21.function b/tests/suites/test_suite_pkcs1_v21.function
index 8f22f20..f7e1e24 100644
--- a/tests/suites/test_suite_pkcs1_v21.function
+++ b/tests/suites/test_suite_pkcs1_v21.function
@@ -18,6 +18,8 @@
     mbedtls_test_rnd_buf_info info;
     mbedtls_mpi N, E;
 
+    info.fallback_f_rng = mbedtls_test_rnd_std_rand;
+    info.fallback_p_rng = NULL;
     info.buf = rnd_buf->x;
     info.length = rnd_buf->len;
 
@@ -122,6 +124,8 @@
     mbedtls_test_rnd_buf_info info;
     mbedtls_mpi N, P, Q, E;
 
+    info.fallback_f_rng = mbedtls_test_rnd_std_rand;
+    info.fallback_p_rng = NULL;
     info.buf = rnd_buf->x;
     info.length = rnd_buf->len;