tests: make rtpaux parallel-runnable

By removing global variables, and dehardcoding udp ports. The
latter was probably the cause for random CI issues, as while
the tests aren't run in parallel there, the hardcoded ports may
not always be free.

Fixes #533
6 jobs from rtpaux_tests_fix_raciness in 35 minutes and 19 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #65801
gst indent

00:00:29

passed #65800
manifest

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  Build
passed #65802
build fedora x86_64

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  Test
passed #65803
check fedora

00:15:59

 
  Full Builds
passed #65804
build android arm64 api28

00:03:57

passed #65805
build cerbero fedora x86_64

00:11:17