The whole testdir/runtest.vim is a bit of a mess. So this is an attempt
to improving the current state. First of all, let's move most of the
code into a single RunTest() function so that it's easier to run this
manually.
Then, let's add the possibility to customize filetype specific tests by
applying filetype specific options. We will make use of it in the next
commit, where the provided java file uses tabs, which cause line
wrapping which will then cause the scrolling to miss some lines when
redrawing (see #14105)
related: #14105
Signed-off-by: Christian Brabandt <cb@256bit.org>
