[gdb/testsuite] Fix gdb.tui/pr30056.exp arrow right test

On aarch64-linux, I run into:
...
FAIL: gdb.tui/pr30056.exp: arrow right
...
because while the intention is to observe the change from:
...
|       20 main (void)                  |
...
into:
...
|       20 ain (void)                   |
...
we're actually looking at another line.

Fix this by looking at the contents of the entire source window.

Tested on aarch64-linux and x86_64-linux.

PR testsuite/33506
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33506