test(tvix/cli): Make the REPL testable

Juggle around the internals of the tvix-cli crate so that we expose the
Repl as a public type with a `send` method, that sends a string to the
repl and *captures all output* so that it can be subsequently asserted
on in tests. Then, demonstrate that this works with a single (for now)
REPL test using expect-test to assert on the output of a single command
sent to the REPL.

As the REPL gets more complicated, this will allow us to make tests that
cover that complex behavior.

Change-Id: I88175bd72d8760c79faade95ebb1d956f08a7b83
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11958
Autosubmit: aspen <root@gws.fyi>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/cli/tests/repl.rs

@@ -0,0 +1,27 @@
+use std::ffi::OsString;
+
+use clap::Parser;
+use expect_test::expect;
+use tvix_cli::init_io_handle;
+
+macro_rules! test_repl {
+    ($name:ident() {$($send:expr => $expect:expr;)*}) => {
+        #[test]
+        fn $name() {
+            let tokio_runtime = tokio::runtime::Runtime::new().unwrap();
+            let args = tvix_cli::Args::parse_from(Vec::<OsString>::new());
+            let mut repl = tvix_cli::Repl::new(init_io_handle(&tokio_runtime, &args), &args);
+            $({
+                let result = repl.send($send.into());
+                $expect.assert_eq(result.output())
+                ;
+            })*
+        }
+    }
+}
+
+test_repl!(simple_expr_eval() {
+    "1" => expect![[r#"
+        => 1 :: int
+    "#]];
+});