tests: Fix races in the read-growing-file test

tests: Fix races in the read-growing-file test

First: the buffer is pushed on the encoding filesink only *after* the
checkpoints, leading to wrong ordering in expectations.

The second one is about EOS being posted on the bus *before* the
stop action is executed, leading to the action never being executed.
That action was plain useless as the default behavior of validate
scenario is to internally create a "stop" action on EOS (unless
disabled by user).

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/968>