Add test and subjects of exercise `delete_prefix`

rust/tests/delete_prefix_test/Cargo.lock

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

rust/tests/delete_prefix_test/Cargo.toml

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+[package]
+name = "delete_prefix_test"
+version = "0.1.0"
+authors = ["Augusto <aug.ornelas@gmail.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+delete_prefix = { path = "../../../../rust-piscine-solutions/delete_prefix"}

rust/tests/delete_prefix_test/src/main.rs

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+// Define the function `delete_prefix(prefix: &str, s: &str) -> Option<&str>`
+// That takes 2 slices of string and returns the string of slice s
+// with the `prefix` removed wrapped in Some
+// If `prefix ` is not contained in `s` return None
+
+// Example:
+// delete_prefix("hello, ", "hello, world")? == "world"
+// delete_prefix("not", "win");
+
+use delete_prefix::*;
+
+#[allow(dead_code)]
+fn main() {
+	println!("{:?}", delete_prefix("ab", "abcdefghijklmnop"));
+	println!("{:?}", delete_prefix("x", "abcdefghijklmnop"));
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn test_delete_prefix() {
+		assert_eq!(delete_prefix("john", "john wick"), Some(" wick"));
+
+		assert_eq!(delete_prefix("ab", "b"), None);
+
+		assert_eq!(delete_prefix("aa", "ab"), None);
+
+		assert_eq!(delete_prefix("á©", "á©ab"), Some("ab"));
+	}
+}

subjects/delete_prefix/README.md

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+## delete_prefix
+
+### Instructions
+
+Define the function `delete_prefix(prefix, s)` that returns the string slice `s` with the `prefix` removed wrapped in Some. If `prefix ` is not contained in `s` return None
+
+### Expected Function
+
+```rust
+fn delete_prefix(prefix: &str, s: &str) -> Option<&str> {
+}
+```
+
+### Usage
+
+Here is a program to test your function.
+
+```rust
+fn main() {
+	println!("{:?}", delete_prefix("ab", "abcdefghijklmnop"));
+	println!("{:?}", delete_prefix("x", "abcdefghijklmnop"));
+}
+```
+
+And its output
+
+```console
+student@ubuntu:~/[[ROOT]]/test$ cargo run
+Some("cdefghijklmnop")
+None
+student@ubuntu:~/[[ROOT]]/test$
+```