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>

tests/validate/giosrc/read-growing-file.validatetest

@@ -20,11 +20,13 @@ meta,
 
 
 crank-clock, repeat=5
+wait, on-clock=true
 wait, signal-name=waiting-data, target-element-name=giosrc
 
 checkpoint
 
 crank-clock, repeat=5
+wait, on-clock=true
 wait, signal-name=waiting-data, target-element-name=giosrc
 
 wait, signal-name=done-waiting-data, target-element-name=giosrc, non-blocking=true
@@ -36,7 +38,5 @@ wait, signal-name=waiting-data, target-element-name=giosrc
 
 checkpoint
 
-# Make sure EOS is outputted now.
+# Make sure EOS is outputted now, the scenario will automatically quit on EOS
 set-properties, giosrc::is_growing=false
-
-stop, on-message=eos