[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.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-05-11  Tom de Vries  <tdevries@suse.de>

	* gdb.base/watch_thread_num.exp: Fix "set debug infrun 1" FAILs.
	Add "set debug infrun 1" commented out.