tests: do not shell out taplo to gather forgotten test files

It would be annoying if forgotten tests wouldn't be reported locally.

Cargo.lock

@@ -4006,6 +4006,7 @@ dependencies = [
  "pollster",
  "rand",
  "tempfile",
+ "toml_edit",
 ]
 
 [[package]]

lib/testutils/Cargo.toml

@@ -29,6 +29,7 @@ jj-lib = { workspace = true, features = ["testing"] }
 pollster = { workspace = true }
 rand = { workspace = true }
 tempfile = { workspace = true }
+toml_edit = { workspace = true }
 
 [lints]
 workspace = true

lib/testutils/src/lib.rs

@@ -26,7 +26,6 @@ use std::process::Command;
 use std::process::Stdio;
 use std::sync::Arc;
 
-use bstr::ByteSlice as _;
 use itertools::Itertools as _;
 use jj_lib::backend;
 use jj_lib::backend::Backend;
@@ -635,38 +634,21 @@ pub fn assert_abandoned_with_parent(
 }
 
 pub fn assert_no_forgotten_test_files(test_dir: &Path) {
-    // We require `taplo` for this check; if it's not installed, that's ok unless
-    // we're running in CI.
-    if !is_external_tool_installed("taplo") {
-        ensure_running_outside_ci("`taplo` must be in the PATH");
-        eprintln!(
-            "Skipping check for forgotten test files because taplo is not installed on the system"
-        );
-        return;
-    }
-
-    // Use taplo to find all the test executable main modules listed in `[[test]]`.
-    let taplo_output = Command::new("taplo")
-        .args(["get", "test[*].name", "-f", "Cargo.toml"])
-        .current_dir(test_dir.parent().unwrap())
-        .stdout(Stdio::piped())
-        .stderr(Stdio::piped())
-        .output()
-        .unwrap();
-    assert!(
-        taplo_output.status.success(),
-        "taplo returned with status {}: {}",
-        taplo_output.status,
-        taplo_output.stderr.to_str_lossy(),
-    );
-    let test_bin_mods = taplo_output
-        .stdout
-        .to_str()
-        .unwrap()
-        .trim()
-        .lines()
-        .map(ToString::to_string)
-        .collect_vec();
+    // Parse the integration tests' main modules from the Cargo manifest.
+    let manifest = {
+        let file_path = test_dir.parent().unwrap().join("Cargo.toml");
+        let text = fs::read_to_string(&file_path).unwrap();
+        toml_edit::ImDocument::parse(text).unwrap()
+    };
+    let test_bin_mods = if let Some(item) = manifest.get("test") {
+        let tables = item.as_array_of_tables().unwrap();
+        tables
+            .iter()
+            .map(|test| test.get("name").unwrap().as_str().unwrap().to_owned())
+            .collect()
+    } else {
+        vec![]
+    };
 
     // Add to that all submodules which are declared in the main test modules via
     // `mod`.