Rename to utils_test to drop stretchr/testify dep

I noticed that the pion-ice package had ~1MB of extra binary size due
to the dependency on stretchr/testify.

Renaming test_utils.go to utils_test.go removes stretchr/testify from
any non-test build and fixes this dependency. The tests still pass and
the package still builds the same.

One can check the dependency graph with goda:

https://github.com/loov/goda

goda graph "reach(.:all, github.com/stretchr/testify/require)" |\
    dot -Tsvg -o graph.svg

Signed-off-by: Christian Stewart <christian@aperture.us>