cue: use more realistic arguments for TestDecode
cue: use more realistic arguments for TestDecode

It passed a pointer to an interfaces for each of the
tests, which is somewhat useful, but not ideal.

Change-Id: I8679d3893050b796d520c3b10328fe2202b1d887
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2713
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue