gdb/tui/testsuite: refactor new-layout.exp test

This commit changes the gdb.tui/new-layout.exp test to make use of a
list of test descriptions, and a loop to check each description in
turn.  There's no change to what is actually tested after this commit.

In future commits I plan to add additional tests to this file, and
this will be easier now that all I have to do is add a new test
description to the list.