pad: Let threads in the test take ownership of a strong reference to their pads

Otherwise it can easily happen that the pad is destroyed before the
thread disappears, as happened sometimes in the test_pad_probe_block_add_remove
test where joining of the thread was done *after* the pad was unreffed
and destroyed.

Fixes gstreamer/gstreamer#339
6 jobs for pad-test_pad_probe_block_add_remove in 33 minutes and 47 seconds (queued for 20 minutes and 39 seconds)
Status Job ID Name Coverage
  Preparation
passed #63780
gst indent

00:00:32

passed #63779
manifest

00:00:23

 
  Build
passed #63781
build fedora x86_64

00:02:58

 
  Test
passed #63782
check fedora

00:16:24

 
  Full Builds
passed #63783
build android arm64 api28

00:03:14

passed #63784
build cerbero fedora x86_64

00:10:13