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_ecdsa.function b/tests/suites/test_suite_ecdsa.function
index 7b1718f..06d9fb6 100644
--- a/tests/suites/test_suite_ecdsa.function
+++ b/tests/suites/test_suite_ecdsa.function
@@ -277,9 +277,9 @@
 
     TEST_ASSERT( mbedtls_ecp_group_load( &grp, id ) == 0 );
     TEST_ASSERT( mbedtls_ecp_point_read_string( &Q, 16, xQ_str, yQ_str ) == 0 );
-    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 );
+    TEST_ASSERT( mbedtls_test_read_mpi( &d, 16, d_str ) == 0 );
+    TEST_ASSERT( mbedtls_test_read_mpi( &r_check, 16, r_str ) == 0 );
+    TEST_ASSERT( mbedtls_test_read_mpi( &s_check, 16, s_str ) == 0 );
     rnd_info.buf = rnd_buf->x;
     rnd_info.length = rnd_buf->len;
 
@@ -340,9 +340,9 @@
     memset( hash, 0, sizeof( hash ) );
 
     TEST_ASSERT( mbedtls_ecp_group_load( &grp, id ) == 0 );
-    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 );
+    TEST_ASSERT( mbedtls_test_read_mpi( &d, 16, d_str ) == 0 );
+    TEST_ASSERT( mbedtls_test_read_mpi( &r_check, 16, r_str ) == 0 );
+    TEST_ASSERT( mbedtls_test_read_mpi( &s_check, 16, s_str ) == 0 );
 
     md_info = mbedtls_md_info_from_type( md_alg );
     TEST_ASSERT( md_info != NULL );
@@ -513,7 +513,7 @@
     memset( sig, 0, sizeof( sig ) );
 
     TEST_ASSERT( mbedtls_ecp_group_load( &ctx.grp, id ) == 0 );
-    TEST_ASSERT( mbedtls_mpi_read_string( &ctx.d, 16, d_str ) == 0 );
+    TEST_ASSERT( mbedtls_test_read_mpi( &ctx.d, 16, d_str ) == 0 );
 
     md_info = mbedtls_md_info_from_type( md_alg );
     TEST_ASSERT( md_info != NULL );