[gdb/testsuite] Fix gdb.tui/long-prompt.exp with read1

When running test-case gdb.tui/long-prompt.exp with check-read1, we get:
...
(gdb) FAIL: gdb.tui/long-prompt.exp: prompt size == width + 1: \
  end of screen: at last line
...

The problem is in these commands:
...
    Term::command "echo \\n"
    Term::command "echo \\n"
    Term::command "echo \\n"
    Term::command "echo \\n"
...

The last one makes the terminal scroll, and the scrolling makes the expected
output match on a different line.

Fix this by replacing the sequence with a single command:
...
    Term::command "echo \\n\\n\\n\\n\\n\\n"
...
which avoids scrolling.

Tested on x86_64-linux.