vim-patch:9.2.0395: tests: Test_backupskip() may read from $HOME (#39417)

Problem:  tests: Test_backupskip() may read from $HOME
Solution: Set $HOME to an empty value, use --clean
          (D Ben Knoble)

Even though we unset HOME, we can see via scriptnames that user files
are still sourced! One of my installed plugins warns when not compiled
with +python3, so this test has a "press Enter" prompt.

Use `--clean` like most other GetVimProg()'s do to fix it. Some tests
use `system()` instead, but that turns this test into a failure rather
than passing; I'm not sure why other tests don't suffer from this.

To prove to ourselves, we can use code like this:

    diff --git i/src/testdir/test_options.vim w/src/testdir/test_options.vim
    index a408e20e1..044364a54 100644
    --- i/src/testdir/test_options.vim
    +++ w/src/testdir/test_options.vim
    @@ -1179,6 +1179,7 @@ func Test_backupskip()
       " P_NODUP).  Run this in a separate instance and write v:errors in a file,
       " so that we see what happens on startup.
       let after =<< trim [CODE]
    +      call writefile([execute('scriptnames')], 'foo')
           let bsklist = split(&backupskip, ',')
           call assert_equal(uniq(copy(bsklist)), bsklist)
           call writefile(['errors:'] + v:errors, 'Xtestout')
    @@ -1196,7 +1197,7 @@ func Test_backupskip()
       " unset $HOME, so that it won't try to read init files
       let saveenv['HOME'] = getenv("HOME")
       call setenv('HOME', v:null)
    -  exe 'silent !' . cmd
    +  exe 'silent !' . cmd .. ' --cmd "echo &rtp"'
       call assert_equal(['errors:'], readfile('Xtestout'))

       " restore environment variables

Here, that causes "foo" to include a bunch of files under ~/.vim. I'm
not sure why this happens, but lets paper over it for the test.

We can also tell that (orthogonal to --clean) setting HOME='' works too.
Let's do that in addition since unsetting HOME isn't quite enough.

closes: vim/vim#20051

https://github.com/vim/vim/commit/8d9c383aaff17f5991f739a6d0f06017022b2198

Co-authored-by: D. Ben Knoble <ben.knoble+github@gmail.com>

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