gdb/testsuite: some additional tests in gdb.tui/scroll.exp

gdb/testsuite: some additional tests in gdb.tui/scroll.exp

This commit just adds an extra check of the src window size prior to
sending all the commands to gdb.  We also set the cmd window height to
its existing height, this (obviously) shouldn't change the window
layout, which we check.

My main motivation was adding the initial window layout check, the
winheight and recheck are just extras.  All of these test pass both
before and after this commit.