test: Improve 008_autocommands_spec reliability (#9129)

Sometimes 008_autocommands_spec fails like this:

    [ RUN      ] autocommands that delete and unload buffers: BufUnload, VimLeave: -- Output to stderr:
    CMake Error at /home/travis/build/neovim/neovim/cmake/RunTests.cmake:53 (message):
      functional tests failed with error: 1

The final :quit + wait() is a race. Use command() instead, which is
synchronous.  Use command('silent! ...') everywhere else too, because
it's clearer instead of feeding input and clearing the expected errors
with CTRL-L.

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