Ignore invalid_target_empty test.

This is due to a change in clap 3.2. Cargo's CI runs without a lock
file, but rust-lang/rust runs with one (locked to an older version). To
avoid issues, this just ignores the test. See
rust-lang#10753 for more.

tests/testsuite/cargo_add/invalid_target_empty/mod.rs

@@ -6,6 +6,7 @@ use crate::cargo_add::init_registry;
 use cargo_test_support::curr_dir;
 
 #[cargo_test]
+#[ignore]
 fn invalid_target_empty() {
     init_registry();
     let project = Project::from_template(curr_dir!().join("in"));