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/cp.rs

@@ -0,0 +1,86 @@
+extern crate codex_execpolicy;
+
+use codex_execpolicy::ArgMatcher;
+use codex_execpolicy::ArgType;
+use codex_execpolicy::Error;
+use codex_execpolicy::ExecCall;
+use codex_execpolicy::MatchedArg;
+use codex_execpolicy::MatchedExec;
+use codex_execpolicy::Policy;
+use codex_execpolicy::Result;
+use codex_execpolicy::ValidExec;
+use codex_execpolicy::get_default_policy;
+
+#[expect(clippy::expect_used)]
+fn setup() -> Policy {
+    get_default_policy().expect("failed to load default policy")
+}
+
+#[test]
+fn test_cp_no_args() {
+    let policy = setup();
+    let cp = ExecCall::new("cp", &[]);
+    assert_eq!(
+        Err(Error::NotEnoughArgs {
+            program: "cp".to_string(),
+            args: vec![],
+            arg_patterns: vec![ArgMatcher::ReadableFiles, ArgMatcher::WriteableFile]
+        }),
+        policy.check(&cp)
+    )
+}
+
+#[test]
+fn test_cp_one_arg() {
+    let policy = setup();
+    let cp = ExecCall::new("cp", &["foo/bar"]);
+
+    assert_eq!(
+        Err(Error::VarargMatcherDidNotMatchAnything {
+            program: "cp".to_string(),
+            matcher: ArgMatcher::ReadableFiles,
+        }),
+        policy.check(&cp)
+    );
+}
+
+#[test]
+fn test_cp_one_file() -> Result<()> {
+    let policy = setup();
+    let cp = ExecCall::new("cp", &["foo/bar", "../baz"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec::new(
+                "cp",
+                vec![
+                    MatchedArg::new(0, ArgType::ReadableFile, "foo/bar")?,
+                    MatchedArg::new(1, ArgType::WriteableFile, "../baz")?,
+                ],
+                &["/bin/cp", "/usr/bin/cp"]
+            )
+        }),
+        policy.check(&cp)
+    );
+    Ok(())
+}
+
+#[test]
+fn test_cp_multiple_files() -> Result<()> {
+    let policy = setup();
+    let cp = ExecCall::new("cp", &["foo", "bar", "baz"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec::new(
+                "cp",
+                vec![
+                    MatchedArg::new(0, ArgType::ReadableFile, "foo")?,
+                    MatchedArg::new(1, ArgType::ReadableFile, "bar")?,
+                    MatchedArg::new(2, ArgType::WriteableFile, "baz")?,
+                ],
+                &["/bin/cp", "/usr/bin/cp"]
+            )
+        }),
+        policy.check(&cp)
+    );
+    Ok(())
+}