Reword test cases

Reword test cases to be easier to read and understand.
Adds comments to better explain what the test is doing.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/tests/suites/test_suite_mpi.function b/tests/suites/test_suite_mpi.function
index 97107c2..6f305e9 100644
--- a/tests/suites/test_suite_mpi.function
+++ b/tests/suites/test_suite_mpi.function
@@ -1166,28 +1166,31 @@
 
 /* BEGIN_CASE */
 void mbedtls_mpi_exp_mod_size( int A_bytes, int E_bytes, int N_bytes,
-                               int radix_RR, char * input_RR, int div_result )
+                               int radix_RR, char * input_RR, int exp_result )
 {
     mbedtls_mpi A, E, N, RR, Z;
     mbedtls_mpi_init( &A  ); mbedtls_mpi_init( &E ); mbedtls_mpi_init( &N );
     mbedtls_mpi_init( &RR ); mbedtls_mpi_init( &Z );
 
+    /* Set A to 2^(A_bytes - 1) + 1 */
     TEST_ASSERT( mbedtls_mpi_lset( &A, 1 ) == 0 );
-    TEST_ASSERT( mbedtls_mpi_lset( &E, 1 ) == 0 );
-    TEST_ASSERT( mbedtls_mpi_lset( &N, 1 ) == 0 );
-
     TEST_ASSERT( mbedtls_mpi_shift_l( &A, ( A_bytes * 8 ) - 1 ) == 0 );
-    TEST_ASSERT( mbedtls_mpi_shift_l( &E, ( E_bytes * 8 ) - 1 ) == 0 );
-    TEST_ASSERT( mbedtls_mpi_shift_l( &N, ( N_bytes * 8 ) - 1 ) == 0 );
-
     TEST_ASSERT( mbedtls_mpi_set_bit( &A, 0, 1 ) == 0 );
+
+    /* Set E to 2^(E_bytes - 1) + 1 */
+    TEST_ASSERT( mbedtls_mpi_lset( &E, 1 ) == 0 );
+    TEST_ASSERT( mbedtls_mpi_shift_l( &E, ( E_bytes * 8 ) - 1 ) == 0 );
     TEST_ASSERT( mbedtls_mpi_set_bit( &E, 0, 1 ) == 0 );
+
+    /* Set N to 2^(N_bytes - 1) + 1 */
+    TEST_ASSERT( mbedtls_mpi_lset( &N, 1 ) == 0 );
+    TEST_ASSERT( mbedtls_mpi_shift_l( &N, ( N_bytes * 8 ) - 1 ) == 0 );
     TEST_ASSERT( mbedtls_mpi_set_bit( &N, 0, 1 ) == 0 );
 
     if( strlen( input_RR ) )
         TEST_ASSERT( mbedtls_mpi_read_string( &RR, radix_RR, input_RR ) == 0 );
 
-    TEST_ASSERT( mbedtls_mpi_exp_mod( &Z, &A, &E, &N, &RR ) == div_result );
+    TEST_ASSERT( mbedtls_mpi_exp_mod( &Z, &A, &E, &N, &RR ) == exp_result );
 
 exit:
     mbedtls_mpi_free( &A  ); mbedtls_mpi_free( &E ); mbedtls_mpi_free( &N );