Fix an intermittent test failure in sslapitest

PR#23659 modified the tls-provider to assign group ids to the various
dummy groups.

The group id for each dummy group was stored in the int * value from
xor_group_params[3].data. This pointer is actually the location of the
global variable xor_group.group_id which is normally used to store the
group id for "xorgroup".

The group data is created by the tls_prov_get_capabilities function which
gets called every time an SSL_CTX is created by the calling application. The
first time it is called the xor_group.group_id has been initialised to the
correct value and xorgroup gets assigned the correct group id- but the
tls_prov_get_capabilities function overwrites it with a different value
before it exits. This means that the second and any subsequent times it
gets called, it starts off with the wrong value and xorgroup gets associated
with this incorrect value.

In the test that fails two SSL_CTX objects are created. They end up getting
different values for xorgroup and the test fails. The failure is
intermittent based on the test ordering. If the tests are run in a different
order then it can happen to work because tls-provider has already been
loaded into the libctx by an earlier test and SSL_CTXs have previously been
created. Therefore the xorgroup value ends up being the same (wrong) value
in the two SSL_CTX values that are created in the test - and the test
passes by accident.

test/tls-provider.c

@@ -396,6 +396,7 @@ static int tls_prov_get_capabilities(void *provctx, const char *capability,
 
         for (i = 0; i < NUM_DUMMY_GROUPS; i++) {
             OSSL_PARAM dummygroup[OSSL_NELEM(xor_group_params)];
+            unsigned int grpid;
 
             memcpy(dummygroup, xor_group_params, sizeof(xor_group_params));
 
@@ -411,7 +412,8 @@ static int tls_prov_get_capabilities(void *provctx, const char *capability,
             dummygroup[0].data = dummy_group_names[i];
             dummygroup[0].data_size = strlen(dummy_group_names[i]) + 1;
             /* assign unique group IDs also to dummy groups for registration */
-            *((int *)(dummygroup[3].data)) = 65279 - NUM_DUMMY_GROUPS + i;
+            grpid = 65279 - NUM_DUMMY_GROUPS + i;
+            dummygroup[3].data = &grpid;
             ret &= cb(dummygroup, arg);
         }
     }