test(add): Remove expensive test

This is a carry-over from cargo-edit where we had to worry about the UX
of all of the behavior while now we are just relying on built-in cargo
behavior and don't need to test it specifically for `cargo add`.

On my machine, this test takes 11s.

tests/testsuite/cargo_add/invalid_git_external/in

@@ -1 +0,0 @@
-../add-basic.in

tests/testsuite/cargo_add/invalid_git_external/mod.rs

@@ -1,28 +0,0 @@
-use cargo_test_support::compare::assert_ui;
-use cargo_test_support::prelude::*;
-use cargo_test_support::Project;
-
-use cargo_test_support::curr_dir;
-
-#[cargo_test]
-fn case() {
-    cargo_test_support::registry::init();
-
-    let project = Project::from_template(curr_dir!().join("in"));
-    let project_root = project.root();
-    let cwd = &project_root;
-    let git_url = url::Url::from_directory_path(cwd.join("does-not-exist"))
-        .unwrap()
-        .to_string();
-
-    snapbox::cmd::Command::cargo_ui()
-        .arg("add")
-        .args(["fake-git", "--git", &git_url])
-        .current_dir(cwd)
-        .assert()
-        .code(101)
-        .stdout_matches_path(curr_dir!().join("stdout.log"))
-        .stderr_matches_path(curr_dir!().join("stderr.log"));
-
-    assert_ui().subset_matches(curr_dir!().join("out"), &project_root);
-}

tests/testsuite/cargo_add/invalid_git_external/out/Cargo.toml

@@ -1,5 +0,0 @@
-[workspace]
-
-[package]
-name = "cargo-list-test-fixture"
-version = "0.0.0"

tests/testsuite/cargo_add/invalid_git_external/stderr.log

@@ -1,12 +0,0 @@
-    Updating git repository `[ROOTURL]/case/does-not-exist/`
-...
-error: failed to load source for dependency `fake-git`
-
-Caused by:
-  Unable to update [ROOTURL]/case/does-not-exist/
-
-Caused by:
-  failed to clone into: [ROOT]/home/.cargo/git/db/does-not-exist-[..]
-
-Caused by:
-...

tests/testsuite/cargo_add/mod.rs

@@ -36,7 +36,6 @@ mod git_rev;
 mod git_tag;
 mod infer_prerelease;
 mod invalid_arg;
-mod invalid_git_external;
 mod invalid_git_name;
 mod invalid_key_inherit_dependency;
 mod invalid_key_overwrite_inherit_dependency;