[gdb/testsuite] Update infrun regexp in gdb.base/watch_thread_num.exp
The test-case gdb.base/watch_thread_num.exp contains an infrun regexp:
...
-re "infrun:" {
...
which doesn't trigger because:
- the test-case doesn't contain "set debug infrun 1", and
- if we hack the test-case to add this, the regexp doesn't match
because "[infrun] " is printed instead.
Make the test pass with "set debug infrun 1" and add the setting commented
out.