test: faster test execution in codex-core (#2633)

this dramatically improves time to run `cargo test -p codex-core` (~25x
speedup).

before:
```
cargo test -p codex-core  35.96s user 68.63s system 19% cpu 8:49.80 total
```

after:
```
cargo test -p codex-core  5.51s user 8.16s system 63% cpu 21.407 total
```

both tests measured "hot", i.e. on a 2nd run with no filesystem changes,
to exclude compile times.

approach inspired by [Delete Cargo Integration
Tests](https://matklad.github.io/2021/02/27/delete-cargo-integration-tests.html),
we move all test cases in tests/ into a single suite in order to have a
single binary, as there is significant overhead for each test binary
executed, and because test execution is only parallelized with a single
binary.

codex-rs/execpolicy/tests/suite/literal.rs

@@ -0,0 +1,50 @@
+use codex_execpolicy::ArgType;
+use codex_execpolicy::Error;
+use codex_execpolicy::ExecCall;
+use codex_execpolicy::MatchedArg;
+use codex_execpolicy::MatchedExec;
+use codex_execpolicy::PolicyParser;
+use codex_execpolicy::Result;
+use codex_execpolicy::ValidExec;
+
+extern crate codex_execpolicy;
+
+#[test]
+fn test_invalid_subcommand() -> Result<()> {
+    let unparsed_policy = r#"
+define_program(
+    program="fake_executable",
+    args=["subcommand", "sub-subcommand"],
+)
+"#;
+    let parser = PolicyParser::new("test_invalid_subcommand", unparsed_policy);
+    let policy = parser.parse().expect("failed to parse policy");
+    let valid_call = ExecCall::new("fake_executable", &["subcommand", "sub-subcommand"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec::new(
+                "fake_executable",
+                vec![
+                    MatchedArg::new(0, ArgType::Literal("subcommand".to_string()), "subcommand")?,
+                    MatchedArg::new(
+                        1,
+                        ArgType::Literal("sub-subcommand".to_string()),
+                        "sub-subcommand"
+                    )?,
+                ],
+                &[]
+            )
+        }),
+        policy.check(&valid_call)
+    );
+
+    let invalid_call = ExecCall::new("fake_executable", &["subcommand", "not-a-real-subcommand"]);
+    assert_eq!(
+        Err(Error::LiteralValueDidNotMatch {
+            expected: "sub-subcommand".to_string(),
+            actual: "not-a-real-subcommand".to_string()
+        }),
+        policy.check(&invalid_call)
+    );
+    Ok(())
+}