Enable mpi_mod_int test case to take full-range MPI integers

Also add commented-out test cases that currently fail

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/tests/suites/test_suite_bignum.function b/tests/suites/test_suite_bignum.function
index 4cec0a7..40b23fe 100644
--- a/tests/suites/test_suite_bignum.function
+++ b/tests/suites/test_suite_bignum.function
@@ -959,24 +959,47 @@
 /* END_CASE */
 
 /* BEGIN_CASE */
-void mpi_mod_int( char * input_X, int input_Y,
-                  int input_A, int div_result )
+void mpi_mod_int( char * input_X, char * input_Y,
+                  char * input_A, int mod_result )
 {
     mbedtls_mpi X;
+    mbedtls_mpi Y;
+    mbedtls_mpi A;
     int res;
     mbedtls_mpi_uint r;
-    mbedtls_mpi_init( &X );
 
-    TEST_ASSERT( mbedtls_test_read_mpi( &X, input_X ) == 0 );
-    res = mbedtls_mpi_mod_int( &r, &X, input_Y );
-    TEST_ASSERT( res == div_result );
+    mbedtls_mpi_init( &X );
+    mbedtls_mpi_init( &Y );
+    mbedtls_mpi_init( &A );
+
+    /* We use MPIs to read Y and A since the test framework limits us to
+     * ints, so we can't have 64-bit values */
+    TEST_EQUAL( mbedtls_test_read_mpi( &X, input_X ), 0 );
+    TEST_EQUAL( mbedtls_test_read_mpi( &Y, input_Y ), 0 );
+    TEST_EQUAL( mbedtls_test_read_mpi( &A, input_A ), 0 );
+
+    TEST_EQUAL( Y.n, 1 );
+    TEST_EQUAL( A.n, 1 );
+
+    /* Convert the MPIs for Y and A to signed mbedtls_mpi_uints */
+    mbedtls_mpi_uint y = Y.p[0];
+    if( Y.s == -1 )
+        y = -y;
+    mbedtls_mpi_uint a = A.p[0];
+    if( A.s == -1 )
+        a = -a;
+
+    res = mbedtls_mpi_mod_int( &r, &X, y );
+    TEST_EQUAL( res, mod_result );
     if( res == 0 )
     {
-        TEST_ASSERT( r == (mbedtls_mpi_uint) input_A );
+        TEST_EQUAL( r, a );
     }
 
 exit:
     mbedtls_mpi_free( &X );
+    mbedtls_mpi_free( &Y );
+    mbedtls_mpi_free( &A );
 }
 /* END_CASE */