rust: Remove libtest from stdlibs list

libtest does not need to be linked for every module type, so remove it
from the stdlibs list. Instead, link it only when building benchmarks or
tests.

Bug: 193782599
Test: cd external/rust/crates/; mma
Test: Rust tests still run correctly.
Change-Id: I536be8754da0987e09340744d9ebf668b8e734d0