gdb: don't duplicate 'thread' field in MI breakpoint output

When creating a thread-specific breakpoint with a single location, the
'thread' field would be repeated in the MI output.  This can be seen
in two existing tests gdb.mi/mi-nsmoribund.exp and
gdb.mi/mi-pending.exp, e.g.:

  (gdb)
  -break-insert -p 1 bar
  ^done,bkpt={number="1",type="breakpoint",disp="keep",
	      enabled="y",
	      addr="0x000000000040110a",func="bar",
	      file="/tmp/mi-thread-specific-bp.c",
	      fullname="/tmp/mi-thread-specific-bp.c",
	      line="32",thread-groups=["i1"],
	      thread="1",thread="1",  <================ DUPLICATION!
	      times="0",original-location="bar"}

I know we need to be careful when adjusting MI output, but I'm hopeful
in this case, as the field is duplicated, and the field contents are
always identical, that we might get away with removing one of the
duplicates.

The change in GDB is a fairly trivial condition change.

We did have a couple of tests that contained the duplicate fields in
their expected output, but given there was no comment pointing out
this oddity either in the GDB code, or in the test, I suspect this was
more a case of copying whatever output GDB produced and using that as
the expected results.  I've updated these tests to remove the
duplication.

I've update lib/mi-support.exp to provide support for building
breakpoint patterns that contain the thread field, and I've made use
of this in a new test I've added that is just about creating
thread-specific breakpoints and checking the results.  The two tests I
mentioned above as being updated could also use the new
lib/mi-support.exp functionality, but I'm going to do that in a later
patch, this way it is clear what changes I'm actually proposing to
make to the expected output.

As I said, I hope that frontends will be able to handle this change,
but I still think its worth adding a NEWS entry, that way, if someone
runs into problems, there's a chance they can figure out what's going
on.

This should not impact CLI output at all.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Pedro Alves <pedro@palves.net>

gdb/breakpoint.c

@@ -6480,7 +6480,7 @@ print_one_breakpoint_location (struct breakpoint *b,
      For the CLI output, the thread/task information is printed on a
      separate line, see the 'stop only in thread' and 'stop only in task'
      output below.  */
-  if (!header_of_multiple && uiout->is_mi_like_p ())
+  if (part_of_multiple && uiout->is_mi_like_p ())
     {
       if (b->thread != -1)
 	uiout->field_signed ("thread", b->thread);