Add -h/--help flag to testbin (nushell#16196)

# Description
As title, this pr introduce `-h` flag to testbin, so if I want to see
which testbin I should use, I don't need to look into source code.

### About the change
I don't know if there is any way to get docstring of a function inside
rust code. So I created a trait, and put docstring into it's `help`
method:
```rust
pub trait TestBin {
    // the docstring of original functions are moved here.
    fn help(&self) -> &'static str;
    fn run(&self);
}
```
Take `cococo` testbin as example, the changes are:
```
original cococo function --> Cococo struct, then
1. put the body of `cococo` function into `run` method
2. put the docstring of `cococo` function into `help` method
```

# User-Facing Changes

`-h/--help` flag in testbin is enabled.
```
> nu --testbin -h
Usage: nu --testbin <bin>
<bin>:
chop -> With no parameters, will chop a character off the end of each line
cococo -> Cross platform echo using println!()(e.g: nu --testbin cococo a b c)
echo_env -> Echo's value of env keys from args(e.g: nu --testbin echo_env FOO BAR)
echo_env_mixed -> Mix echo of env keys from input(e.g: nu --testbin echo_env_mixed out-err FOO BAR; nu --testbin echo_env_mixed err-out FOO BAR)
echo_env_stderr -> Echo's value of env keys from args to stderr(e.g: nu --testbin echo_env_stderr FOO BAR)
echo_env_stderr_fail -> Echo's value of env keys from args to stderr, and exit with failure(e.g: nu --testbin echo_env_stderr_fail FOO BAR)
fail -> Exits with failure code 1(e.g: nu --testbin fail)
iecho -> Another type of echo that outputs a parameter per line, looping infinitely(e.g: nu --testbin iecho 3)
input_bytes_length -> Prints the number of bytes received on stdin(e.g: 0x[deadbeef] | nu --testbin input_bytes_length)
meow -> Cross platform cat (open a file, print the contents) using read_to_string and println!()(e.g: nu --testbin meow file.txt)
meowb -> Cross platform cat (open a file, print the contents) using read() and write_all() / binary(e.g: nu --testbin meowb sample.db)
nonu -> Cross platform echo but concats arguments without space and NO newline(e.g: nu --testbin nonu a b c)
nu_repl -> Run a REPL with the given source lines
relay -> Relays anything received on stdin to stdout(e.g: 0x[beef] | nu --testbin relay)
repeat_bytes -> A version of repeater that can output binary data, even null bytes(e.g: nu --testbin repeat_bytes 003d9fbf 10)
repeater -> Repeat a string or char N times(e.g: nu --testbin repeater a 5)
```

# Tests + Formatting
None, all existed tests can guarantee the behavior of testbins doesn't
change.

# After Submitting
NaN

Author: WindSoilder

src/main.rs

@@ -367,28 +367,18 @@ fn main() -> Result<()> {
 
     start_time = std::time::Instant::now();
     if let Some(testbin) = &parsed_nu_cli_args.testbin {
-        // Call out to the correct testbin
-        match testbin.item.as_str() {
-            "echo_env" => test_bins::echo_env(true),
-            "echo_env_stderr" => test_bins::echo_env(false),
-            "echo_env_stderr_fail" => test_bins::echo_env_and_fail(false),
-            "echo_env_mixed" => test_bins::echo_env_mixed(),
-            "cococo" => test_bins::cococo(),
-            "meow" => test_bins::meow(),
-            "meowb" => test_bins::meowb(),
-            "relay" => test_bins::relay(),
-            "iecho" => test_bins::iecho(),
-            "fail" => test_bins::fail(),
-            "nonu" => test_bins::nonu(),
-            "chop" => test_bins::chop(),
-            "repeater" => test_bins::repeater(),
-            "repeat_bytes" => test_bins::repeat_bytes(),
-            // Important: nu_repl must be called with `--testbin=nu_repl`
-            // `--testbin nu_repl` will not work due to argument count logic
-            // in test_bins.rs
-            "nu_repl" => test_bins::nu_repl(),
-            "input_bytes_length" => test_bins::input_bytes_length(),
-            _ => std::process::exit(1),
+        let dispatcher = test_bins::new_testbin_dispatcher();
+        let test_bin = testbin.item.as_str();
+        match dispatcher.get(test_bin) {
+            Some(test_bin) => test_bin.run(),
+            None => {
+                if ["-h", "--help"].contains(&test_bin) {
+                    test_bins::show_help(&dispatcher);
+                } else {
+                    eprintln!("ERROR: Unknown testbin '{test_bin}'");
+                    std::process::exit(1);
+                }
+            }
         }
         std::process::exit(0)
     } else {