Clean up t0050: 'checkout with no pathspec and a case insensitive fs' to an appropriate place

[Denton-L]

SZEDER Gábor writes here:

I don't like this test ;)

I only intended it as a "here is how to reliably reproduce the
segfault without all the clutter of the full git.git repository" that
I wrote way past my bedtime. But I think that:

• it shouldn't have the CASE_INSENSITIVE_FS prereq. Yes, that
  segfault could only be triggered on a case insensitive filesystem,
  but the given sequence of commands should succeed in a case
  sensitive file system just as well.

  (Have no idea why I added that prereq in the first place; as I
  said above, it was way past my bedtime...)

• it's in the wrong test script; it would be better among other
  tests checking what 'git checkout' should or must not overwrite
  when switching branches, but not sure which test script that is.

  (I think I added it to this test script, because it stood out a
  bit when grepping for case insensitive fs in the test suite; I
  play the "past my bedtime" card again :)

• it's already satisfied by 'git checkout master' not failing, but
  it doesn't check whether the resulting contents of the worktree
  are as expected.

• it still bothers me why that additional subdir was necessary to
  trigger the segfault. Did you look into it?