test: rtpbin_buffer_list: move buffer list creation next to its validation

The tests create a buffer list and then use the chain_list callback to
verify that the correct packets have been pushed.

Move the creation and validation code next to each other so that the
reader can more easily understand what is going on.

While at it add some comments to introduce the two related functions.
12 jobs for <span class="ref-name">rtpbin-buffer-list-test-port-to-GStreamer-1.0</span> in 54 minutes and 49 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #207204
gst indent

00:00:22

passed #207203
manifest

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  Build
passed #207205
build fedora x86_64

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passed #207206
build nodebug fedora x86_64

00:10:04

 
  Test
passed #207207
check fedora

00:02:38

 
  Full Builds
passed #207209
build cerbero android universal

00:14:17

passed #207210
build cerbero cross win32

00:09:38

passed #207211
build cerbero cross win64

00:08:11

passed #207208
build cerbero fedora x86_64

00:08:41

 
  Full Tests
passed #207213
valgrind bad

00:05:44

passed #207212
valgrind good

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  Apps
passed #207214
android universal examples

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