Improve error injection in EC J-PAKE tests

Instead of corrupting the public key part of the message,
corrupt the proof part. A proof is conceptually similar to a signature,
and changing anything in it should make it invalid with 
a high probability.
Also, instead of shifting data, perform a bitflip.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/tests/suites/test_suite_psa_crypto.function b/tests/suites/test_suite_psa_crypto.function
index 36a8efa..779f594 100644
--- a/tests/suites/test_suite_psa_crypto.function
+++ b/tests/suites/test_suite_psa_crypto.function
@@ -790,8 +790,8 @@
 
             if( inject_error == 1 )
             {
-                buffer0[s_x1_pk_off + 8] >>= 4;
-                buffer0[s_x2_pk_off + 7] <<= 4;
+                buffer0[s_x1_pr_off + 8] ^= 1;
+                buffer0[s_x2_pr_off + 7] ^= 1;
                 expected_status = PSA_ERROR_DATA_INVALID;
             }
 
@@ -1013,8 +1013,8 @@
 
             if( inject_error == 2 )
             {
-                buffer1[c_x1_pk_off + 12] >>= 4;
-                buffer1[c_x2_pk_off + 7] <<= 4;
+                buffer1[c_x1_pr_off + 12] ^= 1;
+                buffer1[c_x2_pr_off + 7] ^= 1;
                 expected_status = PSA_ERROR_DATA_INVALID;
             }