Add the exercise `groceries`'s subject and tests

rust/tests/groceries_test/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "groceries"
+version = "0.1.0"
+
+[[package]]
+name = "groceries_test"
+version = "0.1.0"
+dependencies = [
+ "groceries",
+]

rust/tests/groceries_test/Cargo.toml

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+[package]
+name = "groceries_test"
+version = "0.1.0"
+authors = ["MSilva95 <miguel-silva98@hotmail.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+groceries = { path = "../../../../rust-piscine-solutions/groceries" }

rust/tests/groceries_test/src/main.rs

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+// # Instructions
+
+// Create a function called `insert`
+// fn insert(vec: &mut Vec<String>, val: String) that inserts a new element at the end of the Vec
+
+#[allow(unused_imports)]
+use groceries::*;
+
+#[allow(dead_code)]
+fn main() {
+	let mut groceries = vec![
+		"yogurt".to_string(),
+		"panetone".to_string(),
+		"bread".to_string(),
+		"cheese".to_string(),
+	];
+	insert(&mut groceries, String::from("nuts"));
+	println!("The groceries list now = {:?}", &groceries);
+	println!(
+		"The second element of the grocery  list is {:?}",
+		at_index(&groceries, 1)
+	);
+}
+
+#[test]
+fn test_insertions() {
+	let mut groceries = Vec::new();
+	insert(&mut groceries, "milk".to_string());
+	assert_eq!(groceries, ["milk"]);
+	insert(&mut groceries, "bread".to_string());
+	assert_eq!(groceries, ["milk", "bread"]);
+}
+
+#[test]
+fn test_index() {
+	let groceries: Vec<String> = vec![
+		"milk".to_string(),
+		"bread".to_string(),
+		"water".to_string(),
+		"wine".to_string(),
+	];
+	assert_eq!(at_index(&groceries, 0), "milk");
+}

subjects/groceries/README.md

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+## groceries
+
+### Instructions
+
+Create a function called `insert` that inserts a new element at the end of the Vec
+
+And another function `at_index` that returns the value found at the index passed as an argument
+
+### Expected Functions
+
+```rust
+pub fn insert(vec: &mut Vec<String>, val: String) {
+}
+
+pub fn at_index(vec: &Vec<String>, index: ) -> String {
+}
+```
+
+### Usage
+
+```rust
+use groceries::{insert, at_index}
+
+fn main() {
+	let mut groceries = vec![
+		"yogurt".to_string(),
+		"panetone".to_string(),
+		"bread".to_string(),
+		"cheese".to_string(),
+	];
+	insert(&mut groceries, String::from("nuts"));
+	println!("The groceries list now = {:?}", &groceries);
+	println!(
+		"The second element of the grocery  list is {:?}",
+		at_index(&groceries, 1)
+	);
+}
+```
+
+And its output:
+
+```rust
+The groceries list now = ["yogurt", "panetone", "bread", "cheese", "nuts"]
+The second element of the grocery  list is "panetone"
+```