Add unit tests for libgtp gtpie.[ch] functions

This doesn't yet cover all the functions in gtpie.[ch], but testing half
of them is better than not testing any of them, so let's merge this
current state with a couple of TDOO's on what we still need to test.

Change-Id: I30a6dd8a01b7a074ef2d3936d186dfff6c79e6c0
diff --git a/tests/gtp/gtpie_test.ok b/tests/gtp/gtpie_test.ok
new file mode 100644
index 0000000..465df72
--- /dev/null
+++ b/tests/gtp/gtpie_test.ok
@@ -0,0 +1,6 @@
+Testing gtpie_tlv()
+Testing gtpie_tv0()
+Testing gtpie_tv1()
+Testing gtpie_tv2()
+Testing gtpie_tv4()
+Testing gtpie_tv8()