testing/spreed-webrtc: fix ftbfs with go 1.16

go 1.16 is defaulting to go modules, but this project does not provide
any, so provide them ourselves.

There are also some build issues that are fixed.
8 jobs for !22098 with spreed-webrtc-fix-ftbfs in 3 minutes and 26 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Lint
passed #409878
docker-alpine x86_64
lint

00:00:21

 
  Build
passed #409883
docker-alpine ci-build aarch64
build-aarch64

00:01:49

passed #409885
docker-alpine ci-build armhf
build-armhf

00:03:01

passed #409884
docker-alpine ci-build armv7
build-armv7

00:01:55

passed #409882
docker-alpine ci-build ppc64le
build-ppc64le

00:02:16

passed #409881
docker-alpine ci-build s390x
build-s390x

00:01:50

passed #409880
docker-alpine ci-build x86
build-x86

00:01:05

passed #409879
docker-alpine ci-build x86_64
build-x86_64

00:00:54