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
diff --git a/rust/benchmark.go b/rust/benchmark.go
index b89f5cd..0e84243 100644
--- a/rust/benchmark.go
+++ b/rust/benchmark.go
@@ -101,6 +101,7 @@
 func (benchmark *benchmarkDecorator) compilerDeps(ctx DepsContext, deps Deps) Deps {
 	deps = benchmark.binaryDecorator.compilerDeps(ctx, deps)
 
+	deps.Rustlibs = append(deps.Rustlibs, "libtest")
 	deps.Rustlibs = append(deps.Rustlibs, "libcriterion")
 
 	return deps