Migrate tests of cargo-init to snapbox

Some redundant tests are merged or deleted.

- `gitignore_appended_not_replaced` -> `git_ignore_exists_no_conflicting_entries`
- `gitignore_added_newline_in_existing` -> `git_ignore_exists_no_conflicting_entries`
- `gitignore_no_newline_in_new` -> `simple_git`
- `terminating_newline_in_existing_git_ignore` -> `git_ignore_exists_no_conflicting_entries`
- `terminating_newline_in_new_git_ignore` -> `simple_git`
- `terminating_newline_in_new_mercurial_ignore` -> `simple_hg`
- `terminating_newline_in_existing_mercurial_ignore` -> `simple_hg_ignore_exists`
- `mercurial_added_newline_in_existing` -> `simple_hg_ignore_exists`
- `mercurial_no_newline_in_new` -> `simple_hg`
- `cargo_lock_gitignored_if_lib1` -> `simple_git`
- `cargo_lock_gitignored_if_lib2` -> `inferred_lib_with_git`
- `cargo_lock_not_gitignored_if_bin2` -> `inferred_bin_with_git`
- `cargo_lock_not_gitignored_if_bin1` -> `explicit_bin_with_git`

Author: weihanglo

tests/snapshots/init/auto_git.in

@@ -0,0 +1 @@
+empty-dir

tests/snapshots/init/auto_git.out/.gitignore

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+/target
+/Cargo.lock

tests/snapshots/init/auto_git.out/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "case"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

tests/snapshots/init/auto_git.out/src/lib.rs

@@ -0,0 +1,8 @@
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn it_works() {
+        let result = 2 + 2;
+        assert_eq!(result, 4);
+    }
+}

tests/snapshots/init/auto_git.stderr

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+     Created library package

tests/snapshots/init/auto_git.stdout

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+fn main() {
+    println!("Check that our file is not overwritten")
+}
+

tests/snapshots/init/bin_already_exists_explicit.in/src/main.rs

@@ -0,0 +1,8 @@
+[package]
+name = "case"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

tests/snapshots/init/bin_already_exists_explicit.out/Cargo.toml

@@ -0,0 +1,4 @@
+fn main() {
+    println!("Check that our file is not overwritten")
+}
+

tests/snapshots/init/bin_already_exists_explicit.out/src/main.rs

@@ -0,0 +1 @@
+     Created binary (application) package