[gdb/testsuite] Fix timeout in gdb.tui/new-layout.exp

[gdb/testsuite] Fix timeout in gdb.tui/new-layout.exp

In test-case gdb.tui/new-layout.exp we run into:
...
WARNING: timeout in accept_gdb_output
PASS: gdb.tui/new-layout.exp: layout=cmd_only {cmd 1} {} {}: \
  bottom of cmd window is blank
...

The timeout happens here:
...
    Term::command "layout src"
...

Before the "layout src" command we have:
...
Screen Dump (size 80 columns x 24 rows, cursor at column 46, row 7):
    0 +-tui-layout.c-------------------------+(gdb) layout example3
    1 |       20 {                           |(gdb) layout src
    2 |       21   return 0;                 |(gdb) winheight cmd 8
    3 |       22 }                           |(gdb) layout example4
    4 |       23                             |(gdb) layout src
    5 |       24                             |(gdb) winheight cmd 8
    6 |       25                             |(gdb) layout example5
    7 |       26                             |(gdb)
    8 |       27                             |
    9 |       28                             |
   10 |       29                             |
   11 |       30                             |
   12 |       31                             |
   13 |       32                             |
   14 |       33                             |
   15 |       34                             |
   16 |       35                             |
   17 |       36                             |
   18 |       37                             |
   19 |       38                             |
   20 |       39                             |
   21 |       40                             |
   22 +--------------------------------------+
   23 exec No process In:                                                L??   PC: ??
...
and after:
...
Screen Dump (size 80 columns x 24 rows, cursor at column 6, row 16):
    0 +-tui-layout.c-----------------------------------------------------------------+
    1 |       20 {                                                                   |
    2 |       21   return 0;                                                         |
    3 |       22 }                                                                   |
    4 |       23                                                                     |
    5 |       24                                                                     |
    6 |       25                                                                     |
    7 |       26                                                                     |
    8 |       27                                                                     |
    9 |       28                                                                     |
   10 |       29                                                                     |
   11 |       30                                                                     |
   12 |       31                                                                     |
   13 |       32                                                                     |
   14 +------------------------------------------------------------------------------+
   15 exec No process In:                                                L??   PC: ??
   16 (gdb)
   17
   18
   19
   20
   21
   22
   23
...

The Term::command "layout src" is waiting to match:
- "(gdb) layout src", and then
- "(gdb) ".

The first part fails to match on a line:
...
|       26                             |(gdb) layout src
...
because it expects the prompt at the start of the line.

Fix this by allowing the prompt at the start of a window as well.

Tested by x86_64-linux.