tests: rtpcollision: use alawenc/dec in these tests instead of Speex
tests: rtpcollision: use alawenc/dec in these tests instead of Speex

They should always be built, while the speex elements are not.

Need to check for a smaller number of buffers then (7->4) because
speexenc will add 3 header buffers while alawenc will just output
as many buffers as it receives as input.

https://bugzilla.gnome.org/show_bug.cgi?id=742098

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