validate:tests: Fix race in `validate_padmonitor.buffer_outside_segment`

We were using a fakesrc which data flow was potentially breaking the
test.
6 jobs for check.gst-devtools.validate_padmonitor.buffer_outside_segment in 29 minutes and 44 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #74350
gst indent

00:00:12

passed #74349
manifest

00:00:34

 
  Build
passed #74351
build fedora x86_64

00:04:51

 
  Test
passed #74352
check fedora

00:14:58

 
  Full Builds
passed #74353
build android arm64 api28

00:05:29

passed #74354
build cerbero fedora x86_64

00:09:19