tests: Migrate legacy test 61.

This test is real time based as it also tests the `:earlier` and `:later`
commands with time arguments (using `:sleep`).  This can sometimes case the
test to fail on systems that are under heavy load or where the time interval
between creating the expected buffer state and the `:earlier` or `:later`
command that tries to jump to it changes.

To be system independent we use nvim's `:sleep` command and `wait()` for it in
the test suit.

The legacy vim test writes to test.out a lot with `:.w >>test.out`.  This
does currently not work in the lua test suite so the test is modernized to use
busted's assertions instead of the output file.

This test was treated special in the legacy Makefile but after the conversion
the related code can be removed.

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