[gdb/testsuite] Generate long string in gdb.base/page.exp

[gdb/testsuite] Generate long string in gdb.base/page.exp

I noticed in gdb.base/page.exp:
...
set fours [string repeat 4 40]
...
but then shortly afterwards:
...
    [list 1\r\n 2\r\n 3\r\n 444444444444444444444444444444]
...

Summarize the long string in the same way using string repeat:
...
    [list 1\r\n 2\r\n 3\r\n [string repeat 4 30]]
...

Tested on x86_64-linux.