Use mbedtls_test_read_mpi in test suites

Replace calls to mbedtls_mpi_read_string() with a wrapper
mbedtls_test_read_mpi() when reading test data except for the purpose
of testing mbedtls_mpi_read_string() itself. The wrapper lets the test
data control precisely how many limbs the constructed MPI has.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_debug.function b/tests/suites/test_suite_debug.function
index 0e663c6..fe5549a 100644
--- a/tests/suites/test_suite_debug.function
+++ b/tests/suites/test_suite_debug.function
@@ -179,9 +179,7 @@
 
     TEST_ASSERT( mbedtls_ssl_setup( &ssl, &conf ) == 0 );
 
-    /* If value is empty, keep val->n == 0. */
-    if( value[0] != 0 )
-        TEST_ASSERT( mbedtls_mpi_read_string( &val, radix, value ) == 0 );
+    TEST_ASSERT( mbedtls_test_read_mpi( &val, radix, value ) == 0 );
 
     mbedtls_ssl_conf_dbg( &conf, string_debug, &buffer);