test(add): Cover some frontmatter corner cases (#15886)

### What does this PR try to resolve?

When working with the frontmatter parser, I identified some corner cases
that we should cover.

I cannot think of a programmatic way to require escaping when going from
no frontmatter to a frontmatter or to require more escaping than is
already present.

### How to test and review this PR?

Author: weihanglo

tests/testsuite/cargo_add/mod.rs

@@ -147,7 +147,9 @@ mod rustc_incompatible;
 mod rustc_latest;
 mod rustc_older;
 mod script_bare;
+mod script_escape;
 mod script_frontmatter;
+mod script_frontmatter_empty;
 mod script_shebang;
 mod sorted_table_with_dotted_item;
 mod symlink;

tests/testsuite/cargo_add/script_escape/in/cargo-test-fixture.rs

@@ -0,0 +1,12 @@
+---------
+[package]
+edition = "2015"
+description = """
+Header
+------
+
+Body
+"""
+---------
+
+fn main() {}

tests/testsuite/cargo_add/script_escape/mod.rs

@@ -0,0 +1,39 @@
+use crate::prelude::*;
+use cargo_test_support::Project;
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::current_dir;
+use cargo_test_support::file;
+use cargo_test_support::str;
+
+#[cargo_test]
+fn case() {
+    cargo_test_support::registry::init();
+    for ver in [
+        "0.1.1+my-package",
+        "0.2.0+my-package",
+        "0.2.3+my-package",
+        "0.4.1+my-package",
+        "20.0.0+my-package",
+        "99999.0.0+my-package",
+        "99999.0.0-alpha.1+my-package",
+    ] {
+        cargo_test_support::registry::Package::new("my-package", ver).publish();
+    }
+
+    let project = Project::from_template(current_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .masquerade_as_nightly_cargo(&["script"])
+        .arg("-Zscript")
+        .arg("add")
+        .arg_line("--manifest-path cargo-test-fixture.rs my-package")
+        .current_dir(cwd)
+        .assert()
+        .success()
+        .stdout_eq(str![""])
+        .stderr_eq(file!["stderr.term.svg"]);
+
+    assert_ui().subset_matches(current_dir!().join("out"), &project_root);
+}

tests/testsuite/cargo_add/script_escape/out/cargo-test-fixture.rs

@@ -0,0 +1,15 @@
+---------
+[package]
+edition = "2015"
+description = """
+Header
+------
+
+Body
+"""
+
+[dependencies]
+my-package = "99999.0.0"
+---------
+
+fn main() {}

tests/testsuite/cargo_add/script_escape/stderr.term.svg

@@ -0,0 +1,4 @@
+---
+---
+
+fn main() {}

tests/testsuite/cargo_add/script_frontmatter_empty/in/cargo-test-fixture.rs

@@ -0,0 +1,39 @@
+use crate::prelude::*;
+use cargo_test_support::Project;
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::current_dir;
+use cargo_test_support::file;
+use cargo_test_support::str;
+
+#[cargo_test]
+fn case() {
+    cargo_test_support::registry::init();
+    for ver in [
+        "0.1.1+my-package",
+        "0.2.0+my-package",
+        "0.2.3+my-package",
+        "0.4.1+my-package",
+        "20.0.0+my-package",
+        "99999.0.0+my-package",
+        "99999.0.0-alpha.1+my-package",
+    ] {
+        cargo_test_support::registry::Package::new("my-package", ver).publish();
+    }
+
+    let project = Project::from_template(current_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .masquerade_as_nightly_cargo(&["script"])
+        .arg("-Zscript")
+        .arg("add")
+        .arg_line("--manifest-path cargo-test-fixture.rs my-package")
+        .current_dir(cwd)
+        .assert()
+        .success()
+        .stdout_eq(str![""])
+        .stderr_eq(file!["stderr.term.svg"]);
+
+    assert_ui().subset_matches(current_dir!().join("out"), &project_root);
+}

tests/testsuite/cargo_add/script_frontmatter_empty/mod.rs

@@ -0,0 +1,6 @@
+---
+[dependencies]
+my-package = "99999.0.0"
+---
+
+fn main() {}