In 3b12bb225adda2aac40a55f7009cae05311b2a43, ":oldfiles" was taught to
behave like Vim's ":browse oldfiles" if ":oldfiles!" was used.  However,
this conflates the use of ! for abandoning a modified buffer with
choosing one file out of a list of oldfiles.

Now that ":browse" is supported again, ":browse oldfiles" will allow the
user to select an old file, while still complaining if that would cause
a modified buffer to be abandoned.  ":browse oldfiles!" will just
abandon the buffer, as expected.

Cf. https://github.com/neovim/neovim/pull/4995

The test hit wait_return if x or .x.swp exists in the project root directory.

menu_spec.lua yanks to the clipboard, but never pastes from it.  This
can leave a child xsel process waiting around for something to paste the
content, causing the test process to hang.

Since the test isn't explicitly trying to exercise the clipboard, simply
use the default register.

It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.

The directories table contains the names of the expected directory names
for varying scopes of the :cd tests.  Using named indexes, instead of
numbered, makes the test more readable.