gdb/testsuite: fix race in gdb.server/multi-ui-errors.exp

gdb/testsuite: fix race in gdb.server/multi-ui-errors.exp

After this commit:

  commit ed32754a8c7919feffc6ddb66ff1c532e4a4d1cd
  Date:   Thu Mar 9 10:45:03 2023 +0100

      [gdb/testsuite] Fix gdb.server/multi-ui-errors.exp for remote target

I noticed the occasional failure in gdb.server/multi-ui-errors.exp,
which looked like this:

  (gdb) PASS: gdb.server/multi-ui-errors.exp: interact with GDB's main UI
  interrupt
  (gdb)
  Program received signal SIGINT, Interrupt.
  0x00007ffff7d501e7 in nanosleep () from /lib64/libc.so.6
  FAIL: gdb.server/multi-ui-errors.exp: interrupt (timeout)
  PASS: gdb.server/multi-ui-errors.exp: interrupt arrived
  p server_pid
  $1 = 718174
  (gdb) PASS: gdb.server/multi-ui-errors.exp: p server_pid

This is triggered by this code in gdb.server/multi-ui-errors.exp:

    gdb_test "interrupt"

    gdb_test_multiple "" "interrupt arrived" {
	-re "Program received signal SIGINT, Interrupt\\.\r\n" {
	    pass $gdb_test_name
	}
    }

The problem here is that the first interrupt will trigger the prompt
to be printed, and then, after some time the inferior will be
interrupted.

However the default pattern for gdb_test includes a '$' end anchor.
If expect sees the prompt with nothing following it then everything is
fine, and the test passes.

However, if the interrupt is quick and so what expect sees is this:

  (gdb)
  Program received signal SIGINT, Interrupt.
  0x00007ffff7d501e7 in nanosleep () from /lib64/libc.so.6

In this case the end anchor means that the gdb_test fails to match,
and eventually times out.

Fix this by passing -no-prompt-anchor to gdb_test.

Reviewed-By: Tom de Vries <tdevries@suse.de>