[gdb/testsuite] Fix regexp in gdb.ada/mi_var_access.exp

[gdb/testsuite] Fix regexp in gdb.ada/mi_var_access.exp

With gcc-12 and target board unix/-m32, we run into:
...
(gdb) ^M
Expecting: ^(-var-create A_String_Access \* A_String_Access[^M
]+)?(\^done,name="A_String_Access",numchild="1",.*[^M
]+[(]gdb[)] ^M
[ ]*)
-var-create A_String_Access * A_String_Access^M
^error,msg="Value out of range."^M
(gdb) ^M
FAIL: gdb.ada/mi_var_access.exp: Create varobj (unexpected output)
...

What happens is easier to understand if we take things out of the mi context:
...
$ gdb -q -batch \
    outputs/gdb.ada/mi_var_access/mi_access \
    -ex "b mi_access.adb:19" \
    -ex run \
    -ex "p A_String_Access"
  ...
Breakpoint 1, mi_access () at mi_access.adb:19
19         A_String : String (3 .. 5) := "345"; -- STOP
$1 = (pck.string_access) <error reading variable: Value out of range.>
...
while with target board unix we have instead:
...
$1 = (pck.string_access) 0x431b40 <ada_main.sec_default_sized_stacks>
...

The var-create command samples the value of the variable at a location where
the variable is not yet initialized, and with target board unix we
accidentally hit a valid address, but with target board unix/-m32 that's not
the case.

Fix the FAIL by accepting the error message.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28464