Convert rust_libraries and rust_stdlinkage mutators to TransitionMutators

Replace rust.LibraryMutator and rust.LibstdMutator with
TransitionMutators.

Bug: 319288033
Flag: EXEMPT refactor
Test: all soong tests pass
Test: no change to build.ninja
Change-Id: Ia24a582119d39889279d7b93bac9259685153619
diff --git a/rust/rust_test.go b/rust/rust_test.go
index 0d005d0..eeedf3f 100644
--- a/rust/rust_test.go
+++ b/rust/rust_test.go
@@ -60,6 +60,7 @@
 // testRust returns a TestContext in which a basic environment has been setup.
 // This environment contains a few mocked files. See rustMockedFiles for the list of these files.
 func testRust(t *testing.T, bp string) *android.TestContext {
+	t.Helper()
 	skipTestIfOsNotSupported(t)
 	result := android.GroupFixturePreparers(
 		prepareForRustTest,