Fix shellcheck warnings in test_rust.sh

changes/ticket29064

@@ -0,0 +1,2 @@
+  o Code simplification and refactoring (shell scripts):
+    - Fix shellcheck warning in test_rust.sh. Fixes issue 29064.

src/test/test_rust.sh

@@ -16,10 +16,10 @@ for cargo_toml_dir in "${abs_top_srcdir:-../../..}"/src/rust/*; do
     if [ -e "${cargo_toml_dir}/Cargo.toml" ]; then
 	cd "${abs_top_builddir:-../../..}/src/rust" && \
 	    CARGO_TARGET_DIR="${abs_top_builddir:-../../..}/src/rust/target" \
-	    "${CARGO:-cargo}" test ${CARGO_ONLINE-"--frozen"} \
+	    "${CARGO:-cargo}" test "${CARGO_ONLINE-'--frozen'}" \
             --features "test_linking_hack" \
-            --target $rustc_host \
-	    ${EXTRA_CARGO_OPTIONS} \
+            --target "$rustc_host" \
+	    "${EXTRA_CARGO_OPTIONS}" \
 	    --manifest-path "${cargo_toml_dir}/Cargo.toml" || exitcode=1
     fi
 done