[gdb/testsuite] Fix gdb.cp/namespace.exp with read1

[gdb/testsuite] Fix gdb.cp/namespace.exp with read1

With check-read1 we run into:
...
(gdb) break DNE>::DNE^M
Function "DNE>::DNE" not defined.^M
Make breakpoint pending on future shared library load? (y or [n]) y^M
Breakpoint 9 (DNE>::DNE) pending.^M
n^M
(gdb) FAIL: gdb.cp/namespace.exp: br malformed '>' (got interactive prompt)
n^M
...

The question is supposed to be handled by the question and response arguments
to this gdb_test call:
...
gdb_test "break DNE>::DNE" "" "br malformed \'>\'" \
    "Make breakpoint pending on future shared library load?.*" "y"
...
but both this and the builtin handling in gdb_test_multiple triggers.

The cause of this is that the question argument regexp is incomplete.

Fix this by making sure that the entire question is matched in the regexp:
...
set yn_re [string_to_regexp {(y or [n])}]
  ...
    "Make breakpoint pending on future shared library load\\? $yn_re " "Y"
...

Tested on x86_64-linux.